Well I'm sitting in LHR Revivals right now, just off the VS10 from JFK, and there are only 2 other people here.
I guess most passengers who'd be eligible to use it are from the UK and just want to go straight home or to work after a long flight.
But having said that, as far as I can see, the only way an uninitiated passenger would know about it is via the website - and how many people read it that closely?
The obvious place to promote it would be on the UC menu card in-flight, or when (or if) the FSM introduces themselves to individual UC and AU pax. But clearly they've decided not to - maybe they're worried it wouldn't be able to cope, who knows?
I always use it and find Revivals excellent for breakfast, an invigorating shower and often a facial or head massage. I'm then set up for the day and either a car or train to Cambridge or occasionally, meetings in London.
I'm sure it is a bit quieter nowadays than it used to be and I also put it down to the withdrawing of those invites I used to get at check-in.