Mirrorless has pretty much caught up with DSLR now, but only recently. So the worthwhile mirrorless cameras are very expensive (Canon R5, R6 etc). Whereas the older DSLRs like the 1D4, 1Dx etc are relatively cheap and have better image quality (and ease of use for birds and planes) than older mirrorless.
It’s only with the most recent mirrorless options that wildlife / bird photographers have started switching over.
Edit: Also, up until recently, the mirrorless options weren’t designed for sport or wildlife, so even if the IQ was great, the handling, frame rate, AI focus etc would not have been suitable.