To me it makes sense, there are a ton of ethical ramifications to the EMH being sentient that the people of the federation would likely have a very hard time coming to grips with.

Data is a novel one of a kind technology for which the vast majority of people have never seen or interacted with. It's easy to classify him in one way or another as it doesn't effect their life in the big scheme of things.

EMH on the other hand is just a standard hologram, not one created via extenuating circumstances. Meaning the people of the federation would effectively be creating and destroying lifeforms for their own pleasure every time they use the holodeck. I think the modern day equivalent would be to say that every time you turn off your TV or change the channel someone has to die. Or better yet imagine if every time you killed an opponent I'm a video game a sentient life form would have to die.

Possibly a better modern analogue would be the meat and dairy industry. People in modern times commonly accept that dogs are sentient unique individuals with their own personalities, likes, wants and a possibly even a soul. But cows are mindless automatons where it's okay to use them for our pleasure. They aren't 'real' to people the way dogs and cats are. Most people don't want to consider the ethical ramifications of every meal they eat especially when they've been doing things one way for the majority of their life.

If EMH is sentient does that mean they have to stop the use of the holodeck all together? Do they have to "dumb down" the processing of holodeck characters to prevent it from accidentally creating a sentient life? And what would be the ethical implications of all of your holodeck adventures? If you have sex with someone on a holodeck adventure is that considered rape or sexual assault? What is consent if your programmed to feel a certain way from inception.

These are heavy issues for the average federation grunt to have to ponder every time they want to blow off some steam. OR they can just put EMH in the same bucket as every other holodeck character who thinks they are alive but in reality are probably just a few lines of code sitting on the computers storage.