I think part of the reason for the low popularity would be the fact, then HO/VHO have higher arc voltage, so require transformer type of ballast, what is or too inefficient, or too heavy/expensive compare to series choke ballasts, that are sufficient for all other fluorescent lamps on 230V mains. And then the difference between e.g. 65W T12 "normal" and 115W "HO" is assumed as too large to accept the 115W lamp and most better used two 65W "standard" lamps instead.
The 120V area market need transformer ballasts for anything above 30W, so unlike in Europe, people are used to the fact, then ballasts are big and heavy. And then the difference between e.g. 65W "normal" and 115W "HO" is not as significant, so they easily accepted the 115W HO lamp.

I think the difference would disappear in the age of electronic ballasts, where in both market it become common practice in ballast design to first generate ~400..450VDC by the PFC stage and then in the main inverter design is no difference. The reason is, then even in the US the significant part of the professional lighting is supplied from 240 or even 277V, what ask for ballasts being able to fork from all 120, 240 and 277V supply and the easiest way is to cover this in the PFC stage, what require the DC bus to be 450V. This level is still inline with the availability of cost effective components.
But i'm afraid the "good old" HO/VHO disappear in meantime in favor to T5HO...

Here it also turns out that one cannot generalize for the whole of Europe but must differentiate between various countries. So in France and Switzerland HO/VHO lamps saw some use while they are basically unknown in Finland and Germany (or at least East Germany).

Another point: I read that 3500 K is a common light colour for FL lamps in the UK? In Germany it is mostly unknown, with imports from the USSR being the only exception I'm aware of.

I know in Switzerland were in use (and it may be in the France as well - may be related to the cold winter mountain environment in Alps combined with the presence of the "more expensive" tourist sites), but even there not as wide spread as in US.

I wish there was VHO and HO lamps in england!