I think it's at least in part because English £50 notes have, until recently, been relatively easy to counterfeit in comparison to smaller denominations. The Sir John Houblon 50s had no UV ink and no hologram. The genuine notes were often very well worn and very commonly torn across the security threads, in some cases quite dramatically so. They weren't easy to check. By comparison, the new series is more similar to the twenty pound note, and has all the features it does. It doesn't have a hologram, but does have a a very clever "motion thread" that I can imagine being nearly impossible to imitate.
However, the gradual decline in the use of cash isn't helping the fifty either. Lots of transactions over £50 are now done by non-cash methods, and lots of people only use cash for smaller transactions. So there's not perceived to be a need for larger denominations to circulate more widely.
Fifty pounds is not a high denomination, and I think there is a good argument for increasing the circulation of these notes. Fifty pounds a few decades ago was a lot of money, but nowadays it won't cover a typical family's weekly shopping bill. But I don't think there's enough demand to justify it.
I have never had any problems with £50 notes, which I will ask for if for any reason I'm withdrawing a large amount of cash (not very often, unfortunately!). For larger transactions, they are easier to carry around, and I feel safer with a small number of fifties or hundreds than a plethora of twenties. No-one has ever declined a fifty pound note when I've offered it as payment, although I should point out that I use them for reasonable transactions (petrol, shopping, rail tickets etc) and not small things like newspapers, sandwiches or penny sweets.