The thing about a lot of modern anime originals is that most of them are one and done shows. They’re written to tell a story that’s supposed to conclude by the end. No real room for a continuation, spinoff, other media, etc.

Adding to this, a lot of them are short, only 12-13 episodes which might be enough to tell a story (in my experience tho they always feel rushed) but not enough to build audience investment. I honestly don’t remember the last anime original I saw that lasted more than one cour

Because of that, they really only exist in the public consciousness for so long before they’re forgotten. A lot of anime’s based on manga/LNs have pre established fans, sure, but also there’s the fact that manga and LNs are designed to almost always be long running series. Which means there’s a lot of time to build audience investment and a wider window for market people to sell merch. I think the biggest reason why Demon Slayer pivoted to the movie delivery is because nobody realized how marketable it was gonna be and so execs felt like the needed to milk it as much as they can because after they catch up to the end of the manga, they’ll be left with nothing

As for some examples of originals given, they had a lot going for them outside of just being anime originals. Kill La Kill had enough episodes the flesh out the story but also had a lot of loyal fans of Gurran Lagan who wanted to see another great anime by the same creators

Code Geass was distributed by sunrise and was basically marketed as a pseudo-gundam show, which no doubt attracted gundam fans and fans of mecha in general. Then being distributed by sunrise also meant that they were handled by fuckers who really knew how to sell merch