Same reason why people use Google products when they could use something else (and note very often that they can't): it's more convenient because Google products are better. Because Google has the clout to make them better and bury the competition even more. which is the very definition of monopolistic anti-competitiveness.

Element is garbage in my experience. It's just not very user friendly, it's slow, it's bloated (and no wonder, it's a React application) and it's not very stable on the desktop. I tried my best to like it but I just can't: it's awful. And unfortunately, as far as I can tell, that's the best Matrix client out there.

I'm sure the Element people are trying their hardest and I don't fault them. But I'm pretty sure they don't have the resources to make it better, unlike Discord. So people staying on Discord is a self-perpetuating prophecy, until someone commits the resources to make Matrix an easy, fast and attractive proposition.