Because what they did was just add all custom codes they found around internet, plus I guess they didn't even care about the bugs that could appear with some crappy codes.
It's like u take l2jserver and start adding all patches from mxc without testing it 100% and knowing it can't generate bugs.
In the other hand, aCis focused on retail things and solving bugs first, and then they added some custom codes (but not that much, I think), that's why their project is way more stable and good.
And aCis updates > frozen * 50 updates, I guess :poker face:
Plus their developers are way better :poker face: