Theres a number of things.
Its not really a proper sport in the european model, which emphasizes fairness and the importance of rules as driving factors in success..
It doesnt know how to tie into football, which boxing has previously been very well able to exploit.
Its distinctly middle class, and therefore detached from normal britain.
Its distinctly american and therefore detached from normal britain.
Its highly wrestling based and beyond that its martial arts, which doesnt really go down that well in a beer and punch up culture.
Boxing has lineage in the uk, tie ins to football, is easy to understand and visually attractive in a rules sense, is lent legitmacy by the olympics, the bbc and a lot of home grown grass roots fighters.
UFC is visable, and compared to what it is made of "wrestling and karate kid" its doing all right. It is known by a lot of students.
If it wanted to blow up in the UK, its needs to be something that urban kids and pie eating beer swilling english kids watch. It is just too divorced from the real, to get there at the moment.