Because as LadySybil said, it costs a small fortune to stage a decent production. Your ticket price has to cover:
building maintenance and overheads (heat, light etc - do you have any idea how much electricity theatre lights need? It would be cheaper to feed all the lions in London Zoo on nothing but fillet steak and Krug)
staff salaries (from the director down to the poor sod who has to lock up after the audience have gone home and the set has come down)
cast and crew fees, not just for the show you've seen but also for the weeks of rehearsal
agency fees
costumes, set, makeup, props
rehearsal space hire
marketing and advertising
production costs including set design, lighting design, carpenters, joiners,
travel and accommodation if it's a touring show.
A tenner sounds like better value now, doesn't it?
I work in the arts, in a small venue out in the sticks. We get a small grant from the council which just about covers our running costs for 9 months of the year. We have to work really, really hard for our earned income because people a. begrudge paying £14 a ticket for a decent bit of music or drama or whatever and b. getting people to come to something they haven't already seen yet is a real battle. We had a lovely kids show on the other week, but because it wasn't Peppa fucking Pig Live or The Gruffalo, parents were all "ooooh we're not sure about that", like it was going to be a show about kittens being stamped on or something just because they'd never heard of it before. Just give it a chance! Or the smaller companies doing all the exciting and original and cool stuff will fold, and they'll take the smaller venues with them, and you'll be left with massive commercial productions of stuff everybody knows and nothing new or creative. So yes, subsidies are a godsend, and if you're not in the "minority" who benefits, then it's simple - take a chance and buy a fucking ticket ffs!