The first time was early on while still at an exploratory stage. I made the change for productivity reasons. Rails did pay off on that axis, even though I was much less experienced with it than the original stack.

The second time was because I'd gotten it to where it was getting users but I couldn't afford the server costs (given that it was free to use and my users were largely in Vietnam and Thailand). I needed something that could scale much more cheaply.

If I'd had millions of dollars in the bank, I'd have just stuck with what I knew and added people and servers as necessary.