Curiosity at first after it was mentioned a few times by others. This was back in 2007, and I've been off and on with it over the years and being pleasantly surprised with the amount of progress it had made each time I used it.
I switched for good when I built my new PC last year. I didn't actually mind Windows until it began to get filled with crapware, which has really gotten out of control more recently. It's just as well that Proton has eliminated the last reason I needed to use it.