[seanhunter]

A lot of "enterprise" b2b saas systems with relatively low customer numbers, relatively high ticket price per sale are going to be single tenant. Think things like core banking systems[1] which have very sensitive end-customer data (in that case balances and transactions) in them. No bank would be allowed by their regulator to put that in a multi-tenant system even if they would want to which I don't think they would.

Also any system which could notionally be multitenant but the customer is a tech-savvy large enterprise and wants to bring their own cloud. That's de facto single tenant because they're not going to host anyone else's instance are they? So where I work there are a few saas vendors we deal with where we have set up AWS subaccounts where they have some access and they host an instance of their thing in there just for us. Saas vendors will frequently do this if the contract /client is valuable enough, so it's pretty common in an enterprise context.

[1] Mambu, Thought Machine etc