Speaking strictly on kickboxing and not Muay Thai:
Its because the sport has been horribly marketed and mismanaged. Irrespective what some may say, its popularity has been on a steady decline as far as the West is concerned, and Japan.
Not only is it declined from the glory Sport Karate days of the 80-90s, but its declined from the potential resurgance (through K-1) it had in the early 2000s.
As far as Japan, its been in decline starting around the same time PRIDE shutdown (in MMA) up until K-1 itself shutdown.
FEG was a horrible promoter. They held events around the world, but did not bother getting the product on TV. And unlike Pride, did not even bother with an English commentary team. It was on fringe networks like Eurosport and later HDNet where it aired, with their in-house commentary teams. Which few watched.
It is my honest opinion, that Japanese fans got bored of watching the same 10 fighters compete over and over again. At some point you don't want to watch grandpa fight. MAX brought new life, and a Japanese champion, but when Masato retired, it slumped. K-1 should have had weight-divisions like Boxing and MMA, and strived to create multiple champions/stars. They did not. By the time they began the LW tourney, they were already bankrupt.
The WGP tournament format was outdated. It prevented young blood, and Japanese fans were tired of watching the same select fighters fight each other for the 4th or 7th time..
When GLORY started I was very optimistic, they had a gobal vision, got TV deals around the world, created multiple weight divisions. But at some point they just went stale. Even though no WGP, they still held mini tournaments, and small tidbits turned me off. They adopted the MMA weight divisons (Unified Rules) for some reason, as opposed to the Boxing/Kickboxing one. And they ignored the lower weights altogether, who are usually the most bombastic and entertaining. And should be cheaper to afford.