The funny thing being that the impetus for a lot of people moving to Sublime in the first place was that happening to TextMate 1.
Apparently some people, still believe the theory that all you need to know about a product is on the price tag, which is absolutely false (e.g. emacs/vim have no price but are exceptional programmig editors, hardly matched by payware solutions).
Anyway, it's good to have choices.
IIRC -- and I was never a TM user but was following it from the outside -- search for a TM replacement started before the open-sourcing; once TM became popular, people started looking for a cross-platform equivalent (because even lots of people who are primarily OSX users don't do all their editing there) and as the perceptions was that development was stalling, an even more people were searching for alternatives. The open-sourcing was sometime after that.
I don't think ST has been adopted because it is payware (that is, I don't think a significant force in its adoption is a preference for paid over free software), I think its been widely adopted because its features well match what a lot of people are looking for in an editor, and they are willing to pay for it.