It's not easy to adapt for several reasons:
The manga is written for a monthly publication in which the ultimate goal was to provide enough material (plot) for its simultaneous adaptation. This had some problems. For a weekly animation series it wasn't enough because the monthly chapter just offered material for two animation episodes at most.
Sailor Moon Crystal / Eternal proved by adapting the story panel by panel wasn't right either. The ONA format was not a bad idea, I think the problem was to make a 25 minute long episode per act. As an ONA Sailor Moon Crystal season 1 could've had 14 special episodes (one per act) of 45 minutes long by adapting the plot accurately and developing concepts and characters. Also, a film isn't appropriate either due to the episodic structure of some chapters.
Then we have the style and art. The story moves under the dream-like effect of classic shojo and Takeuchi makes it even more ethereal with some confusing events at times. And it's got a lot of monologues. That's almost impossible to translate into animation or live action. The only medium that could adapt that aspect very well would be classical or contemporary ballet. The character designs are not a problem since Utena, for example, also had very stylized designs and was well adapted into animation on a tight budget. However, the Sailor Moon ones are very detailed and excessively ornate so it'd be more practical to simplify them.
And lastly, the manga is obsolete. In the 21st century the story of a group of students fighting against evil while dealing with their normal lives is stale nowadays (and even more so if many of its characters are undeveloped, bar Usagi). It's necessary to explore many characters, including villains, and make more present in their lives (consequences included) that mythological and legendary story pastiche that is narrated in the Dark Kingdom and Black Moon arc, the greek epic poem of Stars, the Biblical and Arthurian elements of Infinity and the Dream arc's Swan lake & Sleeping beauty fairy tales to resonate in today's audiences.