Hey, I'm a completely new user of C3, and here are my two cents.
First of all C3 is up against engines like GDevelop, which I initially got started with. They might not have the same amount of features, but they offer unlimited events for free when developing, and they even offer some free cloud builds. They also have amazing up-to-date high quality tutorials on YouTube that anyone can watch. This is a huge thing for people who are just starting out. And it draws in a lot of people. Not to mention that they do regular game jams, where users can win subscriptions to their game engine. This is a great way to get marketing content for your engine.
Secondly, even though C3 works fine on larger mobile devices, there are no Android and iOS apps for it. You're missing out on a huge ecosystem of young developers.
Finally, the starting price is very steep. Maybe there could be a more limited cheaper version to get started with? A lot of younger people don't have $25 to blow per month on a game engine when there are so many free alternatives. And paying $129 up front is a big ask. GDevelop starts at $5 per month.
PS: A lot of people, especially younger ones, hang out on Discord. I don't think C3 has any official presence there. There's the Construct Community server, but I don't see it marketed anywhere. Maybe I'm wrong. But it was hard to find it for me.
Edit> Resubmitting the post to make some edits, since there appears to be no edit functionality.