Part of it is development priorities. You can't launch a brand new system and hope to immediately take on the whole existing DSLR ecosystem, so mirrorless start

Part of it is development priorities. You can't launch a brand new system and hope to immediately take on the whole existing DSLR ecosystem, so mirrorless started out inhabiting the niches that weren't so well served by DSLRs, because within those niches (at least) some people will be willing to overlook the shortcomings. And then you can build on that, and produce newer models with fewer shortcomings, and maybe start to capture a bigger market.