Understandable, but now I'm going to have to burn everything you've ever loved.
For real though, It's understandable. The only real games in the mainline that are even in English are Nocturne, 4 and apocalypse.
While Nocturne is definitely great, and 4 IMO is a pretty good game as well, they are just two games.(not really talking about Apocalypse cause... it's apocalypse).
It's kinda hard for the mainline to really shine when it only has 2 games openly available to the west, meanwhile there are a bunch of branching series in the Megaten franchise that you can easily give more attention to.
So personally, I really do love the mainline. Hell, I just love Megaten as a whole, but I'd be lying if I said I didn't enjoy all the other games as well. But basically, the TL:DR is that "Mainline is great, but the branching series tend to be the main attraction, and I can't blame anyone for thinking that."