It might be the part of it, but other (maybe main) reason is probably the fact that they had significant crunch culture going and high turnover. They supposedly had trouble recruiting at some point due to running out of interested game dev programmers in Poland. And why take months to train recruits to use your in-house engine, that also requires a lot of work and resources to maintain, when you can use one everyone knows and do simple onboarding.
Heh.