Because any move is traumatic and an emotional wrench, no matter whether the place you are going to is an improvement. Shifting the place you call 'home' from one house to another is emotionally difficult and there's always a place of limbo in the middle where you can't picture the future matching up to the things you are letting go of. However, once you get over the threshold of your new place all these feelings will instantly and significantly shift.
At the risk of getting all pseudo-psychological on you... Could you be transferring some of your left-over grief about your mother's death onto the house? Grief has a habit of spilling out all over the place, with each fresh loss (and moving house is a loss, even if you hate it) triggering memories and emotions from previous ones.