Blink has a younger code base that’s easier to build on. Gecko has been around since the early 90s and has some ancient evils lurking deep within. At least that was the reasoning a while ago. As Mozilla has been putting a heavy emphasis on code correctness for the last few years, that may no longer be the case. Then again, momentum is a big deal, and I still see people saying the don’t want to try Firefox because its memory inefficient even though they fixed that bug almost a decade ago now and its less resource hungry and faster than chrome now