Rock Bottom is still played to this day on classic rock radio (as is Doctor Doctor)...so I’m not buying the “never had a hit” argument.
Not to argue, but TAC is on to something. I grew up with the late 70's, early 80's hard rock "movement", and other than Kiss, which I got into in '77, '78, and Billy Joel, same time, almost all the bands I got into had a hit album or song in the 1980 to 1982 time frame, as played on the radio (for me, WPLR in New Haven, WICC in Bridgeport, and WRKI - "I-95" - in Danbury).
Priest: You've Got Another Thing Coming
Maiden: The Number Of The Beast
Ozzy: Crazy Train (though for me, it was I Don't Know)
Sabbath: Heaven And Hell
Rainbow: Jealous Lover (and later, Stone Cold)
Rush: Tom Sawyer
AC/DC: Back In Black (the album)
Scorpions: No One Like You
I can't speak for anywhere else, but in the Northeast, where I lived, I NEVER heard a UFO song on the radio. The first UFO song I ever heard was "Doctor, Doctor" on either One Night At Budokan, or Rock Will Never Die by MSG (I can't remember which one I got first; the ONAB was an import double and was REALLY expensive, so memory says I got the single disk RWND first).
I jumped to MSG directly because the first album had Billy Sheehan and was produced by Roger Glover (Deep Purple/Rainbow) and the second one had Cozy Powell, also from Rainbow (I had no idea who Paul Raymond was). Even though I was big into Scorpions, I went back into the Roth years, and never quite made the jump to UFO. I just didn't. They didn't open for anyone I saw on tour (though that was a little later, '82, '83, '84, and UFO was on hiatus at that time).