Por que a Coroa permitiu dar Highgarden a Bronn?

The core answer is simple and related to the status of the Tyrell as stewards, not the original descendants of Garth Greenhand. The last hereditary ruler of the Reach, King Mern IX, died at the Campo de fogo, along with his heirs.

• Because the Tyrell's are not the hereditary rulers of Highgarden, Tyrion knows he can install a new Lord.

The Tyrell's have no more hereditary claim than Bronn!

• Olenna relates that of all the great houses of Westeros, the Tyrell claim is the weakest.

"If truth be told, even our claim to Highgarden is a bit dodgy, just as those dreadful Florents are always whining."

—Olenna Tyrell to Sansa Stark

_{A Storm of Swords, Chapter 6, Sansa I.}

"The Tyrells were only stewards that the dragon-kings had upjumped far above their station. Their vanity was exceeded only by their ambition."

—thoughts of Cersei Lannister

_{A Feast for Crows, Chapter 3, Cersei I.}

This is similar to Lord Littlefinger being given Harrenhal--a massive fortress unclaimed by any great Lord. This allows Tywin Lannister to take it from the Whents and give it to Janos Slynt, via Cersei, and Tyrion to subsequently give it to Littlefinger, without resistance or controversy. This awarding of the castle comes with proclaimed control over the Riverlands, an usurpation of House Tully, still a great house but without a link to a mythical founder.

This is mentioned in the show, the books and the supplementary materials. Here's a link from Wiki of Ice&Fire.

At this point in the story, the world has been in a very bloody and long war. One that has destroyed great houses. One that has killed thousands of innocents. Bran's immediate goal is peace.

Why Bronn?

Right now, loyalty is what Bran needs the most in order to fix and rebuild Westeros. Tyrion, Brianne, Davos, and Sam have all been shown to be highly loyal subjects to their respective kings. They have all shown they want a better world.

Bronn, for all his faults, is a smart man. He played the game and managed to rise in the ranks from a common sell sword to a knighthood and eventually a Lord. Most importantly, he is very loyal to whoever pays him the most. He even shot down a dragon and saved Jaime's life when he could have easily ran off. The Crown is paying him Highgarden and a slew of other titles. Nobody else in Westeros can offer him more. His loyalty may be bought, but loyalty is loyalty and that's all that matters.

Can Bronn be flipped?

Bronn has never worked for someone who could not keep his promises. Tyrion and Jaime were both in positions which could pay precisely what they offered. Cersei, at the time, was also capable of paying, but Bronn flipped when it was becoming increasingly apparent that she may not be able to come through after all.

But what could a usurper offer Bronn that he doesn't already have? Hand of the King? That actually seems to be a rather terrible deal. Bronn may not actually quer that title as it has been shown many times how terrible being the Hand can be. Besides, regicide is a difficult task, especially when your target literally knows everything.

At this point, Bronn has everything he wants. He has his lands, titles, and castle. He will work to ensure he keeps all of it.

I really don't think there is any logical reasoning behind why they did this.

I think David and Dan (showrunners) just didn't care or put much thought into it. I think they gave him the title just to have that scene with him sitting at the table with the King's council at the end.

Unless you're going to tell me that every major and minor House in the Reach was killed I don't see any logical reason to give Bronn Highgarden. He has no men following him or a House. He has no loyalties to anything except gold. I am sure they could find a person to lead the Reach and agree to help rebuilding the capital. He literally threatened Tyrion for that position and Tyrion while he is the King's Hand I don't think he is dumb enough to trust Bronn with a title like holding Highgarden.

And that doesn't even start to question if any of the minor houses in the Reach to allow someone like Bronn to take over. I have a hard time believing they would allow a sellsword to simply take over one of the more wealthy castles and holdings in all of Westeros.

Even if all the major and minor Houses in the Reach were killed I still have a hard time believing that other major and minor houses throughout the South half of westeros would even agree to Bronn getting Highgarden. Maybe they would be ok with giving Bronn Harrenhal since giving Harrenhal is treated as somewhat of a joke since it is passed around so often, if I am remembering that right from the books.

Overall it doesn't make a lick of sense. The showrunners love Bronn and gave him more screen time than he probably needed. It was just the showrunners wanting him in the last scene to get a laugh from the audience.

The only surviving Lords or Ladies of the Reach will be of houses that either:

1. Fought for Olena and House Tyrell defending Highgarden against the Lannisters, but surrendered quickly enough to survive, then fought with the Lannisters against Dany and co, but surrendered again, bending the knee to Dany the first time they were asked.
2. Betrayed Olena and House Tyrell by joining Randyll Tarly and the Lannisters, to (as Dickon Tarly described it) fight and kill people people they'd grown up with, hunted and suppered with; then, surrendered to Dany at the first opportunity and bent the knee.
3. Somehow managed to hide from or avoid these wars completely.

It's probably fair to say that anyone in these category is not exactly cream of the crop - and it would be easy for someone politically astute like Tyrion to make a compelling case that they are not someone to trust.

Tyrion could make a case that Bronn is smart, effective, and, if well rewarded, reliable - and if not well rewarded, dangerous. He can make a case that any rival is untrustworthy or weak.

Bran, if interested, can use his powers to confirm that this is true - but also, King Bran isn't particularly interested in the detail of politics. He walked out of the small council meeting at the first opportunity, and put Tyrion in charge of political wrangling as a punishment.

In the absence of any better candidates, it seems reasonable that Tyrion could make this case.