Absolutely this. When cuda was first making headway it was the only thing even remotely close to a "developer environment" and made things significantly easier

Absolutely this. When cuda was first making headway it was the only thing even remotely close to a "developer environment" and made things significantly easier than any of the alternatives. It might be different now, but at that time, many of the users were not computer scientists, they were scientists with computers. Having an easier to use programming model and decent debugging tools means publishing more results, more quickly.