Simple, money.
It's like the reball centre that opened somewhere, people were screaming out for a reball centre which opened and then had to close due to a lack of support. Personally, if there had been a reball centre within 2 hour drive of my house I'd have been there 3 nights a week and again at the weekend.
An astroturf field is ridiculously expensive, and why would any site owner put one up for ballers who want to pay next to nothing for paint, and even less for a site fee, only to turn up and abuse it? Theres a great one now up in ScotKart near edinburgh, we'lll see how much use it gets...