Probably the same reason that more people don't invest into Crypto, they don't understand it. Bloggers seem to want to install/make a quick word press site, generate some hits, and apply for Google adsense. The later option does not take too much effort in creating, and adsense pays in Fiat for easy transfer too a bank account. Ultimately, I think it comes down to convenience.
But this does not make sense. OP is asking why blockchain writing is posted on medium and not steemit... they are already talking about blockchain why not a decentralised platform?
But you're right I think. You can make more money using traditional medium plus ads than with steemit. Steemit seems a lot of spammers and useless posts. Pretty girl picture = $500. Tech writer =$0.01 haha. So it's not so much about crypto or anything, but revenue. Trust me, pretty girl on Medium = $0.