The fact is that, maybe except the UK, and itīs a big maybe, TOS was never as popular as TNG or even Voyager in many European countries or Australia and New Zealand.
In New South Wales, Australia, it's very, very hard to compare. TOS aired once
, in the 60s. Because of the way half of the episodes were rated ("Not Recommended for Children"; only 40 were "G"), TOS never got those "early prime time" repeats that the US got with syndication all through the 70s and 80s (and that Australia experienced with ubiquitous shows like "Lost in Space", "Bewitched" and "Gilligan's Island"). We finally got prime time reruns of TOS "for the first time in colour" in the hiatus between TMP and ST II.
The TOS movies all did very respectable business in Aussie cinemas, AFAIK. In Sydney and Melbourne, we had a cinema screening of five TOS episodes per month on the big screen. For several decades.
When TNG came to most international markets, it was presold to the home video rental
sector. (Sell-thru was very expensive.) A year later, prime time airings were attempted, but the series didn't rate highly enough and ended up as late-night TV fodder (where it won its timeslot, thrilling advertisers).
DS9, VOY (and ENT) were buried even further into the late-night void. And home video sell-thru twin packs.