Quote from ShaDoWValve has already explained quite a lot of these things, in a subtle way, ingame.
Though, they have to maintain a certain balance between story, conversations and the gameplay itself. So far they've been keeping that up quite well in my eyes. There could have been indeed, explained more through events or things you stumble upon in the game... but this might make things a bit too complicated after a while for players less immersed in the Half-Life universe.
Good point though.
In my opinion they were a little too vague with the dialogue and could have told at least A BIT more about what had happened since the first game etc. instead of leaving it ALL to the player's imagination. Ah well..
edit: I agree with most of what curman and taylor said.