The Dolomite had sufficient sporting success.
The real problem is most owners, current and past, tend to talk them down.
I don't think the owners talk them down, the owners are realistic about 40+ year old cars. They aren't desperately trying to convince themselves their trailer-queen £25k RS2000 will out drag a new Golf GTI because they remember their old one in 1991 being a rocketship.
Fords have always had a cult following, many of the current owners are people who had them when they were cheap bangers in the 1980s and 90s, they appealed to the go-faster stripe, spot light, cheap and fast crowd that the Dolomite was always marketed against
. The Dolomite was never a properly cool car for the youth of yesteryear, the OHC was marketed towards executives who wanted a bit of sporting prowess and the OHV were dated family/giffer transport.
The main bodyshell dates from 1965 and when parked next to mid 1970s cars it shows. While it is a smart Italian design it is also upright and tall, by 1980 it was laughably dated.
On top of this Triumph cars are tarred with the BL brush, common pub knowledge that suggests that every car that rolled out of a BL factory was crap by design and made crapper by poor build quality. Some of this probably has some merit but a well maintained BL motor is generally a sound enough car. Dolomites lack the infamous reputation of more radical BL designs such as the Allegro, Princes and SD1, they are hampered by their dullness. A car which has largely faded from memory, and usually the memory that comes back is when somebody watched Sid and Dorris from across the road try and change a wheel and the jack went through the sill of their 1500 in 1985...
Besides, Dolomite values are steadily rising (and roadworthy examples increasing). I bought an MOT'd and roadworthy 1850HL for £850 in 2014. Good luck doing that now!