My apologies if this was already explain but...

[morruin]

It can be frustrating. Just now I spent a little more than 1 hour in the queue for Garuda normal. It pops and the healer apparently clicked the wrong Garuda. But he decides to help us - and procedes to get one-shotted twice during the last phase (1st time leading to a wipe at 2%)- and then leaves. And tomorrow I probably have to spend another hour in the queue to try again.

As for why - how many people would actually opt to join a trial in prgress? You gain nothing compared to a new group, and you risk finding out just why the person you're replacing left.