Its all about the money, money, money. If they took it to say Burnham and Berrow the infrastucture does not fit for the required usage/money.
TOC (as you know) has been on a 5 year rota, just slightly altered for this the 150th. If they went every 10th year it would give others a bit more perhaps but with (I think) 10 courses now on the rota I guess that's enough. I quite like it more frequent there as a personal choice.