Monstrously big volumes are a pain to carry around and a pain to hold and read, plus you miss on awesome covers.
It all comes down to wanting a product that's easier on the wallet or a product that's easier to handle for me, as such I hate what I perceive as the American love with the omnibus format
I dont think its a love for the omnibus format, its just whatever is available. Its not like death note, loveless, neon genesis evangelion, bleach, naruto, one piece, air gear, negima, HANA-KIMI , skip beat, fma, kekkaishi, and a few others didnt or dont have single volume releases out there. Though there are some that only release in two volume omnibus (Blood Lad), but some that are just in omnibus since the original publisher is gone (loveless).