Chrome and Firefox both hold about 20~% of the market (depends at what studies you look at and for what country/ area but they're very close.)
The reason Chrome is likely not supported by the AV is unclear... not sure what you mean.
A lot of the time it's not that the AV doesn't support Chrome it's just that Chrome doesn't support the AV. For example, spybot S&D can't "immunize" Chrome but that's because Chrome doesn't let it... it's not S&D's fault.
That's just one example. Chrome is VERY closed and the program doesn't like to be messed with.