This is a weird change I've been noticing. Before Gen 7, they ran Recover. Roost has the same PP but makes the [email protected]' Levitate ability useless for a turn, and the Lati twins aren't Flying types so they can't take advantage of Roost's positive effect.
As far as I can tell, the answer is that there's no real reason. Latios and Latias learn Recover at level 10.
Source: https://pokemondb.net/pokebase/91537/whats-the-difference-between-roost-and-recover and experience