Latios has more offensive stats
Specifically, that base 130 Special Attack is used to fire off strong Draco Meteors.
That said, as you can see, everything else is more or less identical, there are far too few differences in moveset to account for the tier difference. So, it doesn't make such a huge difference in terms of utility. Both can use Defog and have access to several other support moves that make them so useful. So, with that in mind, it wouldn't make sense for both to be in the same tier. It feels self-contradictory, but if both Pokemon have access to the same utility, wouldn't you choose the one that can fire off a stronger attack?
Latias' special bulk is also wasted, as its typing is not that great defensively. On the other hand, Latios can capitalize on the typing and stat, firing off STAB Draco Meteors and Psyshocks to counter the threats like Scarfed Garchomp, Swift Swim Kindgra, and M-Lopunny and Keldeo respectively.
Lastly, a point I'd like to make is that despite the similarities, their roles are somewhat different. Latias is expected to switch in, use Support moves (or Calm Mind + Stored Power, but that's just one case) and then use Healing Wish.
Latios on the other hand is expected to demolish something, specially as a scarfed revenge killer, something Latias doesn't have the firepower to pull off. If it is against a Wall, it will use support moves and switch out.
In general, Latias is kept till it reaches low health, and then it uses Healing Wish, whereas Latios is switched out, mostly to get rid of the -2 Special Attack after Draco Meteor.
As it usually is, the metagame is more offense based, specially in OU (bulky/hyper offense is more common that balanced which is more common than stall) due to the number of strong wallbreakers, something Latios fits well into.