1. The FATE has a spawn timer of 24-72 hours. Typically on Excalibur, the spawn seems to regularly happen around 12-4 am, which isolates a number of the players.
2. Fate credit. Ixion itself has a set enmity table. This leads to getting limited credit (i.e. You could be in a group and kill Ixion from 100% to 0%, yet ONLY get silver credit (so you only get 1 horn; which leads to my next point).
3. Purchase cost versus horn gathering. Even if the mount drop outright, you may still buy it for 12 horns. The issue is that you have to fight through the sever lag when the FATE is trying to compensate for a "few" people; it makes it difficult to get a fair reward. Silver/bronze is the typical reward level, even if you are there for the entirety of the fight.

Unless you are lucky; you have to wait for it, hope you don't get locked out of the zone/crash, get invited to groups in which you MIGHT get silver credit if you're lucky, and do it 12 times if you are on a populated server.