In browsers at least, DRM doesn't work if your full page doesn't use HTTPS.

The encrypted media extensions (EME) are only available on secure contexts, which means the page needs to be HTTPS. Segments are typically fetched with standard XHR/fetch, which means they need to use HTTPS on secure origins as well. You really don't want to have to deal with mixed content.

Unrelated to DRM, Apple's ATS on iOS kinda requires developers to only use HTTPS origins in their apps.

At that point, you might as well just do HTTPS everywhere.