Just gonna share how I feel about this, some of it from personal experience.
New players, after playing for some time and getting used to the mode, often want to make their own map, because it looks "easy". Compared to osu!standard, there's almost nothing to map: there’s just the circles, and the hitsounds. You don’t need to worry about slider shapes, new combos, combo colours, distance spacing, and all those extra things that make osu!standard mapping more complicated. So Taiko mapping looks easy, and maybe a way to get their first ranked map.
But often they’re a Futsuu or early Muzu player, and so they have no knowledge of Onis. When you’re far away from ever playing Onis, you don’t know how they’re made. So you follow every sound you can hear in the song, and put a note of random colour to it. You don’t know about 1/4 patterns, and what should be red and what should be blue. You make your 1/4 patterns consist of random selections of red and blue, because you think that’s how Onis work. The result is a map that experienced players will not be very happy with, because it’s not very good, but the mapper doesn’t know that. You might’ve made a decent Kantan or Futsuu, but you’re going nowhere if that Oni in your set isn’t of rankable quality.
Experienced players and modders look down upon your Oni, and, depending on how much detail the modder goes into, the mapper is either going to be left confused (“What’s wrong with my Oni? It looks fine to me. And what is this ‘lack of structure’ you’re talking about?”) or they’re going to see that their map isn’t very well liked, and lose faith in ranking it. Then they either get disheartened and choose not to bother mapping anymore, or believe that the system is somehow flawed, and again don’t bother mapping anymore.
Then there’s, as has been already stated, the belief that BNs only focus on maps by famous mappers or BNs. I’m not saying it’s true, but currently it can certainly feel that way - go to most of the BNs profiles and you will see “Not taking requests at the moment.” But at the same time, you see they’re modding other BNs’ maps in their post history. You feel the system is corrupt, it’s impossible to get your map ranked, why bother. So the only people left to keep mapping are the ones who can be sure they’ll get their map ranked.
On a side note, there’s also mod queues. You go to the Modding Queues subform and almost all of the queues are osu!standard only. There are very few for Taiko, sometimes none at all. You check every day and maybe you find one place to ask, and they mod your map. Cool! But finding a constant flow of modders is far harder than it is for osu!standard - especially when you go every day and start seeing the same queues that you’ve already requested in. Recently, there’s been more queues available - but it was far more stagnated in the months previous to this. Some are going to get annoyed about how frequently they have to check the queues list to find a mod. And I think this is even harder sometimes, when there are few queues but a lot of mappers wanting to get mods - when a queue opens, sometimes it can be flooded in minutes.
From the playing perspective, transitioning from Muzukashii to Oni can be very hard. You can be an amazing Muzu player, getting FCs on the >3.75 star Muzukashiis, and now you’re out of Muzukashiis to play. So now you want to try Oni. You may have excelled at 3.75 star Muzukashii, but you can’t immediately do 3.75 star Onis. The different patterns and the increased note density make it far harder than what you’ve experienced in most Muzus. So you have to take a step back - try 3.00 star Onis, then gradually advance forward, to 3.5, and onward. But this, again, can make you feel rather sad, because you’ve had to take a step back in order to progress forward. And getting up to 3.75-4 star Onis can take a long time. If you’re someone to be focused on pp, you’re going to see it stagnate, because a large factor of it is the SR, and you’ve gone down in SR to play Onis. Again, that can make you feel a little annoyed at progression.
Anyway, that’s just some thoughts of mine.