They were pretty much the only game in town, other than IBM and smaller mainframe vendors, if you wanted actual written, binding, guarantees of performance with penalty clauses. (e.g. with real consequences for system failure, such as being credited back X millions of dollars after Y failure)

At least from what I heard pre-HP acquisition, so it’s not ‘amazing lock in’, just that, if you didn’t want a mainframe and needed such guarantees, there was literally no other choice.