It has happened over and over, another soap takes on a couple of actors who were super popular on one soap, figuring they will draw the fans, create the same chemistry....and they don't. I don't watch other soaps, but remember the soap mags gushing over Steve Nichols & Mary Beth Evans (Patch & Kayla) coming to GH, however they just did not have the same dynamic there as on Days. We certainly have seen it on Days, as well liked actors from another soap take on a character on Days, and it just does not work.
I agree Diana Colville was a nice person, but ....people do change as they age, at least most do. Perhaps his mother did not like the fact that Leo was gay, or maybe she regretted having to leave John/Roman, became bitter. Maybe Leo's father was a huge jerk to her, or maybe she just always resented having gotten pregnant with Leo, and took it out on him. Whatever happened, we will have to wait and see. Heck, maybe Leo has some sort of trust fund or inheritance his mother won't let him have.