Because HTML in emails is a notoriously painful process to get working even remotely well.

Not all email providers support HTML in the same way. Some examples:

* At one point Gmail did not support style tags and all styles had to be inline. * Outlook for years used the Word HTML processor which had all sorts of limitations. Hell to get a button you had to create it in VML and put a conditional "if Microsoft do this" on it. * Some (up until a few years ago) versions of Outlook ignore either padding/margin on CSS (forget which one) * In order to layout your email so that it works across all clients, forget about using anything remotely modern. You have to use tables everywhere like it's 2000 all over again.

I'm probably forgetting a bunch, it's been a couple years since I had to deal with it.

With all these issues, adding yet another thing onto it is just asking for pain.