Because you have to jack the engine up pretty high for it to line up with the mount - don't forget it settles quite a lot under the weight and when it's disconnected, the engine drops backwards like crazy........
which you've probably already experienced!
If any more probs, unbolt the rubber (2x 15/16mm bolts through the subframe) and fit it to the mount first.
But from what I can see you can't fit it to the mount first, as the bulkhead and anti-roll bar are in the way. You need to seat the rubber mount onto the sub-frame and then try to line up the bolt holes with the bracket attached to the block...?