your post kind of hints at it, but doesn't really say it: +hit is also better than +crit because some spells cannot crit. warlocks have many spells that cannot crit (corruption, fear, banish, any curse) so they receive no bonus at all from +crit.

+hit is also helpful because you'll have more consistent damage. when you cast a frostbolt you expect it to hit, but not necessarily crit. if it crits you may pull aggro. if you focus on +hit instead you'll do more damage overall and you can more accurately predict what will happen when you cast a spell.