Other RG manage to get their offers out, despite not having Oxbridge levels of funding. Why can't Exeter?
But other courses at other "top" universities also often give decisions on the late end (mid March or even later), so why single out one particular course at Exeter? Typically late decisions are made by courses which are heavily in demand: it's not uncommon to wait for students to make decisions about Oxbridge etc before finalising offers to borderline candidates i.e. to wait for their "top" candidates to decide on firms and insurances, which often happens before the March deadlines.
Sometimes late decisions are made simply because admissions teams are utterly overloaded, through (unexpected) growths in student applications. I know of one place where this happened this year because they launched a new MSci programme and suddenly had masses more applications than they were prepared for, and they couldn't recruit a new member of the admissions team in time. So offers are indeed apparently going out very late for this subject.
It's not fair to compare with Oxbridge for multiple reasons, one being that the Oxbridge knows every year that (home) students will accept a large majority of offers, while other universities have much more complicated calculations to make about what fraction of offers will be accepted. This is another valid reason for late decisions, as they need the information of how many students are firming and insuring with them before deciding to make more offers.
The surprise in the OP's case is that the course wouldn't be expected to be that much in demand, but who can know whether it is more in demand this year than in previous years without inside knowledge.