Because original musicals go down SO well at the moment. People were lining up round the block for
Marguerite! The New London was crammed to the brim with punters for
Gone with the Wind and
Imagine This! And need I remind you of the tremendous roaring success of
Too Close to the Sun?
Just done a quick rundown of the listings at TheatreMonkey. I count eight musicals/plays-with-music that have jukebox scores, versus thirteen musicals with specifically-written scores (although it's early in the morning, so apologies if my count is off). The original musicals column may be clogged with old shows, revivals, and Broadway transfers, but it still wins.
Jukebox musicals may not be to everyone's taste, but wouldn't you rather see a theatre filled with unoriginal fun that keeps cast, crew, and theatre staff in work, as opposed to empty houses that maintain everyone's artistic integrity but don't put food on the table?
Besides! We may go to the theatre all the time, but the theatre isn't just for us who see as much as we can and support the artform in any way possible. It's also for the once-a-year punters who want to have a great night out at
Mamma Mia! or
Jersey Boys. They may not go as often, but the success of the shows they go to speaks VOLUMES for their importance as audience members!
I don't know why I'm so defensive of jukebox musicals lately. I'm sorry.

But if
Rock of Ages wants to come to the West End, and if people want to see it, then there's absolutely no reason to behave like that is a terrible terrible thing. It's not like there's only three theatres in the entirety of London and we're going to lose out on a whole mess of genius by this one show setting up shop for a while.