There are probably licensing issues too. I don't think any commercial copies of this guitar have been made without Brian May's approval. You've also got issues with items like the trem and bridge. You could substitute a standard Tune-O-Matic bridge (though it wouldn't look quite the same) but the trem is pretty unique. The PBG factory don't do anything like it at all. Even the JM models have a standard Strat trem rather than the Jaguar/Jazzmaster style trem. You'd also get Strat style pickups rather than the Burns style ones.

Electrics wise, I'm sure the factory could provide a pre-wired scratchplate which would make it really easy to put together, though the extra time involved in the wiring would add to the cost of the kit.

You've also got the issue of a shorter 24" scale, 24 fret neck and the special dot-markers, which obviously can be done, but again, it's unique to that guitar so the cost goes up.

Of course there's nothing to stop you making your own like Brian did.