Most of those 90% of vendors are not big enough to pull it off. The ones with the muscle to do it successfully are apparently offered special deals by Google that make it not really worth it for them to spend the effort to try and invest in building their own store. Specially if doing so compromises that deal.

Add to that the technical hurdles of trying to run a store in an OS managed by the competition and with increasingly tight security restrictions for functionality that is considered "system level" (eg. automatic updates on F-droid don't work unless you root/flash the firmware..), to the point that you need to make your own OS/firmware if you want to be a real alternative with the same level of user friendliness.

Then add the technical hurdles of installing/managing an alternative firmware for several phone models, to the point that it might be easier to become (or partner with) a phone manufacturer.

Then add to that how competitive and ruthless the phone manufacturing market is, with very thin margins, and how reluctant people are to trying something that isn't already mainstream and doesn't have the fancy apps from the remaining 10% of successful big companies in the Play Store.

A giant as big as Amazon tried to pull it off at a few of those levels (from running their own installable store on regular Android to making their own devices with their own firmware) and even with all the pull from Amazon it isn't making much of a dent. And in some of the device categories (like the fire phones) they already gave up.