Por que Eärendil nasce homem e Dior nasce elfo?

Eärendil was certainly half-Elven

We know Eärendil (and by extension Dior) had to be half-Elven to make the choice, this is because the Valar cannot grant immortality, and unlike Tuor, no intervention from Eru occurred here.

Immortality and Mortality being the special gifts of God to the Eruhini (in whose conception and creation the Valar had no part at all) it must be assumed that no alteration of their fundamental kind could be effected by the Valar even in one case: the cases of Lúthien (and Túor) and the position of their descendants was a direct act of God.
_{The Letters of J.R.R. Tolkien - Letter 153: To Peter Hastings}

Given the above, the quote appears to be due to one of a few things.

• It is either an oversight on Tolkien’s part (possible, but uncommon),
• It could also suggest Eärendil believed trespassing on Valinor would be punished my death and that he would have to deliver his message before the punishment,
• Eärendil being greatly fond of men was unaware he was half-Elven.

An oversight

While unlikely, Tolkien, in his ever changing drafts, may have forgotten or mistaken the line for Eärendil forgetting that he is one of the head-Elven who remain Elven up until their choice, Eärendil could not have had the gift of death taken away from him as that would require a power greater than the Valar and he could not have the gift given to him for the same reasons.

Pena de morte

Given the times, the exiled Noldorin and Men were forbidden to return to the Blessed Realm, as such it may have crossed Eärendil’s mind that in going in search of his parents he may have to enter Valinor, and in doing so would have to relay the message to the Valar before he is executed for his trespassing.

Unaware of his heritage

This is the least likely and probably flat out wrong, but I’ll entertain the idea none-the-less. It may have been the case the Eärendil believed he was of the race of Men, given his preference for them, and in doing so had forgotten his heritage

Finally, no clear statement is made, as far as I could tell, about what was meant by “ere he died”, I guess it’s left to the judgment of the reader.

I am going to go about this in a roundabout way but I promise the end will feature a solid answer.

There are 3 instances of the fates of the Children of Ilúvatar changing. The first two are different than the last.

The first is after Beren and Lúthien are returned to Middle-earth after capturing the Silmaril and they dwell on Tol Galen. Lúthien chooses to become mortal and go where Beren goes after death. (Timeline note: After they die the Silmaril is brought to Dior.)

The second is less certain and deals with Tuor. As he was aging towards death he decided to sail into the West with Idril. Officially, they are never heard from again in Middle-earth. According to Tolkien (letter 153) it’s only conjecture that his fate was joined with the Firstborn.

The third is after Eärendil, breaking through the ban with the power of the Silmaril on his brow, visits the Valar and pleads for them to save Middle-earth. At this point Eärendil, Elwing, Elros and Elrond are allowed to choose for themselves which kindred they are to be joined. The ability to choose their kindred was a decision of Manwë, who was allowed discretion on the matter by Illúvatar himself. Here is the quote from the Silmarillion:

But when all was spoken, Manwë gave judgement, and he said: 'In this matter the power of doom is given to me. The peril that he ventured for love of the Two Kindreds shall not fall upon Eärendil, nor shall it fall upon Elwing his wife, who entered into peril for love of him; but they shall not walk again ever among Elves or Men in the Outer Lands. And this is my decree concerning them: to Eärendil and to Elwing, and to their sons, shall be given leave each to choose freely to which kindred their fates shall be joined, and under which kindred they shall be judged.'

Now to return to the matter of Dior at some point in this sequence of events after Dior received the Silmaril he was killed attempting to defend the realm of Doriath, and his two sons Elured and Elurin were left to starve in the forest. After that Elwing took the Silmaril with her down to the Mouths of Sirion where eventually she flew it out to Eärendil's ship and they continued on to Valinor.

If you stop at the published version of the Silmarillion, thats all the information there is. He died before Manwë gave the choice to the other half-elven, and nothing is said of his fate after his death. However in an earlier version of the Silmarillion, published in The Lost Road e outros escritos, Manwë says something a little different before his decree:

Now all those who have the blood of mortal Men, in whatever part, great or small, are mortal, unless other doom be granted to them; but in this matter the power of doom is given to me. This is my decree: To Eärendel and to Elwing and to their sons shall be given leave each to choose freely under which kindred they shall be judged.

That's pretty definitive and it squarely places Dior and his children among mortal Men before the decree of Manwë which allowed choosing.

That's the best extant, text supported answer I could find.

Personally I feel, given more time that Tolkien would have found a more satisfying simpler answer for this question had he been able to further revise his work.

I suppose it is possible that Dior also was by default a Man,

I think the sobriquet "Thingol's heir" shows that he was by default an Elf.