I think maybe the Rebels decided to make a "last stand"? It's not very clear how much time passes from when our heroes arrive on Yavin 4, and when the Death Star shows up. Possibly just enough time to study the plans, form an attack plan, and brief the pilots.

In Empire, it seems to take at least a day to pack up and prepare to flee Hoth. You don't want to leave stuff behind you're going to need later.

Also, it's clear from Jedi the entire Rebel fleet is not on Yavin 4. They may have wanted to make the Empire think all the eggs were all in one easily to blow up basket.

Yavin is like Jupiter. It's huge, mostly gaseous, and potentially combustable. Blowing us a gas giant you're in orbit around is probably a bad idea. See the end of 2010, for reference. ;)

Alderaan was used as an "effective demonstration" already.