I agree, they are very good unis* and purely being relatively new doesn't mean they can't be. What I meant was more that they haven't done anything to distinguish themselves from other very similar unis so that people will recommend (for example) Warwick over York.
So yeah, I would agree it may well be similar universities not having been in vogue with the same people that use TSR and not having cultivated the word of mouth reputation.
* I would argue that the vast majority of UK unis, to the extent that maybe only one or two aren't, are good unis ... but that's a different discussion.
... and the ones that won't