Rain on the lower slopes of Whistler does not seem that unusual except in maybe the dead of winter-- I haven't been there since about 2004 or so, but...
...even then, everyone at the resort was shaking their heads and saying, "It didn't used to be this way."
I remember skiing all the way to the bottom during my one trip there, which I think was in March. The conditions on the lower slopes were almost unskiable, never been in slush that deep. I had the same feeling I had at Baldy in So Cal, or at Mt. Waterman-- by 2012 or so, that resort could only open maybe a dozen days a year, and even then it was insanely dangerous-- mud, rocks, slush, grass, dirt-- it was a labor of love and nostalgia by the people running it. (Unfortunately, plenty of dirty diesel engines for the lifts as well.) "This place was designed and graded when the weather was very different."
Not citing this as empirical proof of anything-- of course there's plenty of that elsewhere-- just saying, there are a lot of resorts now where I have that feeling, most skiers I know feel the same way, and it's pretty spooky and unnerving.