In SC2, players have more control over every minuet mechanic (constructing buildings, resource mining and management, controlling minions...) in the game, thus putting more tactical responsibility on the burden of the player. In DOTA2, the player is only in control of the super-powered hero itself, and not much of the other aspects of the gameplay.
It is debatable if these options make the game "better" or more difficult as a result. But it is for certain that overall the search space of the problem increases much faster as the dimensions of freedom increases.
Of course, DOTA2 contains very many game mechanics as well (a ton of items that changes a lot of different stats, very many types of heroes which each have their own attacks, a variety of buildings scattered around the map, a shop for player items), but it seems that most of this complexity is focused around player engagements, which although crucial for the development of the game, lets us analyze a much shorter timespan of PvP as opposed to a whole game.
And indeed, the DOTA2 bot from OpenAI was restricted to a mid-lane fight as a singular hero with restricted items, hence restricting most of the complexity DOTA2 has to offer.
Perhaps I am overstepping with this analogy, but the OpenAI result is a bit like a solved chess endgame configuration, while SC2 would be like chess under the knowledge that neither players can see each other's pieces until they are in a position where you can capture them.
In short: SC2 is more tactical. DOTA2 is more arcade-like.