That's six years obsolete, without security patches. What sector has 20% of users with that hardware?
Half of all Android manufacturers suck, and many users are lazy to update, so if you target the platform, you always have to support a bunch of random old versions.
Mainly people in southern America and a few Asian places.
It's likely to get worse as CAs rotate roots more frequently. Cross-signing will work for a time (provided you correctly install) but at some point, older devices will drop out of support and that'll be it.