I really think price often has a lot to do with it. Not just ticket price, but the expense of being in the West End and travel to and from the theatre.
I love musicals!, its the £60-ish for a decent seat that stops me going .... saying though i've managed to go 4 times this year, with another two shows lined up.
I like to go a few times a year. Usually low budget independent stuff. Although I'm not averse to a nice shakespeare production.
Plus in the summer in Glasgow they do bard in the botanics which is a Shakespeare play in the botanic gardens. It was King Lear this year and it rained. If you are going to a small theatre and seeing independent company productions the price is actually pretty reasonable.
I have to agree with the price issue, can cost anywhere between £20-£40 to get down to London (usually only find out with a week or so before that i'm going!), i usually stay the night so there's the cost of somewhere to stay (again, booking week before doesn't help!)... then there's the tickets!
More often then not, i go to the tkts booth which does reduce the cost but still looking at £37 + for tickets, which, sometimes due to where the seats are - i decide just to pay full price and get a better view (being v short)
Suppose that is the downfall of London theatre! Luckily i live near Stratford so i see quite a few RSC productions and having worked there, sometimes get discount - compared to London though its extremely cheap!