Gandalf explica isso para Frodo em mais detalhes no livro, no Capítulo I.2.
“It was not Gollum, Frodo, but the Ring itself that decided things. The Ring left him.”
“What, just in time to meet Bilbo?” said Frodo. “Wouldn't an Orc have suited it better?”
(...)
“The Ring was trying to get back to its master. It had slipped from Isildur's hand and betrayed him; then when a chance came it caught poor Déagol, and he was murdered; and after that Gollum, and it had devoured him. It could make no further use of him: he was too small and mean; and as long as it stayed with him he would never leave his deep pool again. So now, when its master was awake once more and sending out his dark thought from Mirkwood, it abandoned Gollum. Only to be picked up by the most unlikely person imaginable: Bilbo from the Shire!”
Gollum não tem muitos visitantes. O Anel poderia ter arranjado para ser pego por um duende, mas teve menos incentivo até a época da visita de Bilbo, e (isso é especulação da minha parte, mas eu não o inventei) Bilbo era mais provável de ser eficaz o Anel, já que ele era obviamente um viajante de longe que iria viajar ainda mais, ao contrário dos goblins que poderiam ter mantido o Anel nas cavernas sob as Montanhas Sombrias.
Gandalf suspeita que há mais no comportamento do Anel: pode ter sido determinado por Ilúvatar , que assim teria intervindo para causar o fim da Terceira Era, agindo de uma maneira muito mais sutil do que quando sua remodelação do mundo marcou o fim do Segunda Idade . Não me lembro de nenhum material canônico sobre isso.
“Behind that there was something else at work, beyond any design of the Ring-maker. I can put it no plainer than by saying that Bilbo was meant to find the Ring, and not by its maker.”