Percona built a business on 'their' version and operate on the RedHat business model. As such they need a blessed copy for their business model to function.
MariaDB is basically a bid to take control of MySQL back by one of the original MySQL developers.
Etc.
Tbh, the main problem is Oracle has been a terrible steward of MySQL which has led to this mess.
MariaDB was supposed to be a drop-in replacement, but they always had much grander designs than Percona or Webscale do, so they found it was no longer in their interests to track upstream so closely. So after 5.5, MariaDB was going to jump straight to version 10.0. 5.5 is still the last stable release of MariaDB, though, even though Oracle MySQL hit 5.6 over a year ago.
I think "terrible steward" is way off base. 5.5 and 5.6 have both been good releases with some pretty significant improvements for real users, and there's even more stuff coming in 5.7. My general impression is that MariaDB doesn't have the resources to keep up with what Oracle is doing technically, and I'm betting that technical merits will win out.
Also, I read somewhere that Google is contributing engineers to work on Maria. Although how many and for how long (compared to the GOOG's internal databases) remains to be seen.
http://mysqlha.blogspot.com/2012/08/less-open-source.html etc. etc.
You are free to hold a separate and equally valid opinion. :)
Not to make excuses, but oracles position of supporting releases for 8 years on many platforms is not an easy one. Being able to see what works out in branches that don't have this constraint is actually great for the whole ecosystem.
(Disclaimer: former percona employee. Current oracle employee)