Because it's kind of a culturally irrelevant property of a bygone era that hasn't had any significant cultural relevance since the 1980s? How do you reintroduce this property to a new generation and people will get it and flock to it? It's different than the things that are popular right now. Parts of the He-Man franchise are also arguably very dated, hence why they probably think they need to revamp the premise a little bit by having Adam stuck on Earth for most of his youth and the start of his adulthood.
Listen. I love He-Man. I love Masters of the Universe, but I'm just being realistic here. Movies like this are tough to pull off. Yes, Skeletor and He-Man are iconic, but the last live-action He-Man thing was in 1987 and that movie was not a success. The animated CG reboot got canceled. The Kevin Smith shows did better, but the first season had all that stupid controversy. Revolution was way better, but Netflix only gave them five episodes for that one. We have no idea if we're getting another one.
When you look at TMNT, another property that started in the 1980s, TMNT has had way more reboots over the years with varying degrees of success, but it's had more successful reboots than MOTU between the 2003 series, the 2014 live-action movie, the 2012 animated series, and now the Seth Rogen movies. Plus, TMNT has had a stronger presence on toy shelves over the years compared to MOTU.