It looks like neither the IC or the HI in Sofia allow more than three guests in one room (and even in those cases, only for certain suites, not regular rooms).
I suspect this has less to do with maximizing “revenue” and more to do with local regulations related to hotel room occupancy.
Same in a few other IC's .... IC Sandton same and indeed is to do (in that instance) with local regs, but the uplift is modest in that instance. Rates seemed rather good when I booked but was a vanilla configuration.