[asharp]

It's more of a capability thing. If you're running, say, Piston cloud you're using ceph over ethernet to back your disks, so you can easily decouple disk usage and ram usage. If you're stuck using local disks (ie. rackspace/joyent/linode/amazon to a point/etc.), then it's a lot harder to provide that sort of product.

That being said there are providers out there that sell it, and have been for years.

[stephenr]

I’m very aware there are, and we use one of them.

How is it harder (technically. I realise billing is more complex)? You have some software that provisions a VPS with the requested resource limits.