Budget. To use a UK example, Red Dwarf used models for years, mainly because the BBC gave it a standard sitcom budget which it then had to try creating sci-fi sets, props, effects etc with. Then in series 8 (the last of it's initial run), they switched to mostly CGI. Why? Because they found a wunderkid CGI guy who did the effects for them really cheaply. While suitmation is a technique that has survived longer in Japan, that's simply down to economics. Suitmation took over from stop-motion because it was cheaper and quicker rather than because it was "better" (The Beast From 20,000 Fathoms looks and moves much more like a real creature than the stiff, ungainly Godzilla that appeared a year later). In the last year or so the price of CGI has crashed and now it's within the ability of many more TV shows around the world to use it regularly.
I'm a big fan of suitmation and modelmaking (if you ever need someone to gush about how awesome the model south pole base in 'Gorath' is, I'm your man :thumbs

but they are techniques, not genres. The JJ Abrams Star Trek movies are not a different genre to the original series because the former used CGI and the latter used models.
It seems to me that the Japanese somehow have a very different perspective that they bring to their TV shows and films (and indeed possibly to all their art). I'm sure there are people on these forums much more familiar than I with Japanese culture, who could tell you what combination of differing social values, unique local mythology, and everything else that contributes to that. But it's there.
Horror fans have recognised this, and J Horror or Japanese horror seem to be gaining recognition as it's own sub-genre.
Animation fans have recognised this and have repurposed the word "anime" to label cartoons created through this perspective.
IMO, science fiction and fantasy created through this perspective is "tokusatsu". Even if Garo and KR and all the others swapped suits for CGI, as long as they retain that unique perspective, they would still be tokusatsu (whether they'd be better or worse is a completely different matter).