One of the main factor is the number of participants, and in how many countries.
Even in Holland/Belgium, there really are not that many CX riders. Just a lot of good ones. Same with Ice skating, we have some super fast dudes and chicks here, world class, but I don't know one skater personally that really treats it as a sport like I do mtb racing (my main gig). Yet, skaters are quick numerous over the world. Lots of ice stadiums out there. And tv viewers also care about that sport in many of those countries, as long as the countrymen win from time to time.
Even in Belgium, the number of serious MTB racers is still multifold of cyclo-cross, I'm sure. MTB's are so much lower-entry. And you can actually ride them comfortable on trails. Even in the nicer parts of Belgium, CX bikes more or less reduce you to a hiker. Not too many riders that think it's worth it to invest €20000 in a couple bikes and do nothing but CX racing all winter.