It surprises me how many indie devs avoid some of the higher level / more popular engines for this reason alone. But I assume they just must enjoy that sort of stuff much more than I.
The problem with indie devs is purely a lack of knowledge and resources. They don't feel comfortable testing and packaging binaries for distribution on Linux. A decent number of them are also self-taught and actually have almost no exposure to desktop Linux at all. So it's actually a much higher hurdle than you think.