I don't know why. 2-pass never worked for me, ever.
However, I did some experimenting and comparison 1-pass DVDFab vs. 2-pass with identical codec settings using two different programs... my conclusion - 1-pass conversion using DVDFab settings produces far better quality than 2-pass using other programs. But this is my subjective opinion. I'm pretty sure some of the hard-core codec tweaking enthusiasts will jump to argue, but for the average amateur user 1-pass "slow speed, high quality" conversion settings should work just fine.
Of course, you have to remember that not all source content coded equally in a first place. In order to get best results, you have to experiment with conversion tools DVDFab provides - use different codecs, different profiles and presets...
Most of DVDFab users are mainstream users - "one setting fits all" kinda users. It works for most...
Once in a while enthusiasts shows up on these forums and complaints that DVDFab doesn't give them menu of codec tweaks they use to see in other programs... too bad, it doesn't and you have to be an advanced user to go to guts of DVDFab and tweak there... but this is subject for separate discussion.