Products that made a lot of money but...

could have made even more if they'd just been a better.  I remember someone once suggesting the Rausing family would have made even more if people had actually been able to open Tetra Pak cartons.

I was tempted to say Dyson but on the other hand the fact they're so overcomplicated and regularly break just out of warranty has probably helped the Dyson coffers with people buying more hoovers.

zx spectrums   -  wildly successful in uk because of ease of progamming meant a stellar array of games but if the hardware had been better could have swept the US as well as Europe

Betamax

Sony wouldn't allow the adult entertainment industry to use their technology so they opted for VHS who didn't care.

At least that's how I was told the story.  Betamax was far superior in quality but the tapes were shorter so films had to be edited down.  I first saw the Blues Brothers on Betamax and watched it hundreds of times, now when I watch it, I'm still surprised at what appear to be new scenes (deleted to fit on the tape)