The writing has something to do with it. Not that it's bad but it doesn't have well defined arcs which makes it less marketable. I feel like at one point the story must have been heading for a definitive ending and then the plan changed.
The "Chronos Japan" arc is very brief in the manga. Then almost too much happens in the Mount Minikami story line. One right after the other we are introduced to Lost Numbers, Hyper Zoanoids, a proto-zoalord, a true Zoalord, and then the original Zoalord.
The lore introduced is really cool but limits where the series can go. We know all about the Zoanoid / Zoalord hierachy, there are only 12, and who the strongest is.
The Guyvers get sidelined as Aptom vs Team 5, Guyout vs Murakami/ Archanphel takes place. The Guyvers seem to have fewer victories in the manga because of this.
The OAV remedies some of this by moving Team 5 to season 1 and giving the Guyvers a big and exciting battle at Chronos Japan. Presumably also moving Zoalords for a 3rd season.
Of course I think X-Day was also very limiting to the story. The plot kind of meanders and tries to create mystery by bringing back characters believed to be dead and bringing in non canon characters. Not really a good sign.
