EDIT: I'm talking in general. I mine litecoins (and sometimes other alt coins), not bitcoins, so that's what I'm most familiar with.
Big pools are popular for a number of reasons:
1) Lots of hashing power means lots of blocks found means more payment more often, even if it is less coins. People get impatient and leave smaller pools if they aren't getting paid right away.
2) Network effect. It's the first Google search result. This might be the biggest factor.
3) Probably more reputable, so I'll probably get paid too, most pools have honest operators but some can shut down shop and take coins people haven't cashed out yet and run or do other shady things.
4) Probably less downtime, beefier hardware.
I don't know the specifics of GHash in particular if there's anything that makes it more appealing.
