Part of the thing with a lot of yuri, especially the yuri that Morinaga writes, is that it's aimed at a relatively young age range, it's for teens and young adults, this ain't josei. And stories for that kind of age range don't -really- tend to innovate a ton, not because they suck necessarily, but because a new audience is constantly coming of age and experiencing these things for the first time themselves and want to read fiction that speaks to their experiences.
Hana and Hina is still that basic sort of yuri story structure of girl meets girl, there's confusion over whether they like each other or not, and it'll no doubt end with them starting to date. But it's still going to speak to its readers about dealing with fears of coming out, not being sure if someone you like will be homophobic towards you if they find out, and trying to maintain a public image versus being true to what you like. And where once yuri dealt with these issues in a very roundabout "oh it's just adolescence!", winkwink kind of way, writers like Morinaga shifted that into being much more undisguised in its sexual themes and romances.
Granted Morinaga is now starting to get outdone by a whole new generation of even more explicitly queer mangaka, but that's more a demonstration of how quickly the younger generation's culture is changing in regards to homosexuality than it is that Morinaga is like, hopelessly behind the times or something. And for a lot of people in less progressive areas Morinaga's work is still going to speak to them.