Being able to attempt to buy a house is subject to the availability to empty plots. Ex trials can be started at any time.
A single attempt to buy a house can take up to 12 hours. An ex trial takes what, 15 minutes?
And last but not least: You're guaranteed a mount after 99 clears of the ex trial (so after 25 or so hours of farming, maybe some more to account for inevitable wipes), regardless of whether you ever won a roll for it or if it even dropped. There's no such guarantee for a house.
