As contrasted with OpenType? I can't speak for the developers of the fonts but when I compile, Type1 is what I happen to use. I note that the documentation for the MLModern fonts say: OpenType support is planned for a future version.
Yes, as contrasted with otf/ttf. I think of Type-1 as obsolete technology, not often used for new fonts and only supporting 256 glyphs per font. But I guess there must be some upside that I don’t know about.
EDIT: Do you have any insight into why people still work on packages for type-1 fonts? Who still wants to use these, and why?