One of the least helpful questions when discussing TV is, "why would they make this?" It is meant to convey incredulity at perceived poor decision making on the executive level. That what is being discussed is either the product of psychedelic insanity or out and out malice. As many people asked of Agents of S.H.I.E.L.D. when it was being made, "Why would they make a superhero show with no superheroes?" But the fact is that most of the time the answer to that question is usually incredibly boring and pragmatic. In Agents of S.H.I.E.L.D.'s case, it was that Disney wanted to spread Marvel's cinematic success to its struggling network, ABC, without jeopardizing any potential movie franchises. So you wound up with a TV show set in the Marvel Universe that studiously avoided the Marvel Universe for most of its first season.
Da Vinci's Demons is a show that feels like people should be asking "why would anyone make this?" In fact, at the time of its release, people did ask exactly that. But the answer to that would be the most boring answer possible. If someone had made a computer algorithm to create a TV show for Starz, then Da Vinci's Demons would probably be that show. It's a historical conspiracy thriller with elements of fantasy thrown in. This is entirely inline with Starz other contemporary programs, Spartacus and Camelot. And it is enfolding pretty obvious elements into itself: The Da Vinci Code, The Borgias, The Tudors, Assassin's Creed.
In fact, if there was a "most obvious historical figure to turn into a premium cable drama series" Leonardo Da Vinci would be it. He was a genius painter/inventor/scientist/mathematician who lived long enough ago that the period can be exoticized. And, of course, he was a white man (always important). The only problem is that Leonardo was likely gay. In fact, a quick perusal of research on the subject would turn up quite a lot of discussion as to whether Leonardo was celibate or actively homosexual, but no one seems to argue that he slept with women. Which is bit of a consternation for a premium cable drama, where the main character is supposed to be a white man who sleeps with many women.
In the history of television there has been essentially one non-heterosexual male lead of this kind of show, Captain Jack Harkness of Torchwood. When a non-heterosexual man is the central character of a show it is almost universally (to the point where I can't think of an exception other than Torchwood) a non-genre show set in the present about being gay, and most of the time a comedy (Will and Grace, Queer as Folk, Looking, Please Like Me, Transparent). In a grand (and expensive) historical drama, gay characters are meant to be side characters, to show people are sexually libertine or self-hating or oppressed or even just flavoring. Sometimes this is handled very, very well, Spartacus, but these are meant to be the stories of heterosexual, white men. The historical dramas of The Tudors and The Borgias are merely the same male anti-hero drama formula as Breaking Bad or The Sopranos, simply set in a different time. Mad Men even shows there is no line dividing them by being explicitly both. The historical dramas actually have a natural advantage as they are about Popes and Kings, whereas the contemporary versions merely posture in that direction.
Grappling with Leonardo's sexuality is a central problem in the pilot episode that the show can never quite figure out how to resolve. Within the first ten minutes there are two separate scenes detailing gay dalliances. In both of them, one of the two characters dies. It seems to be the show trying to signal that it isn't really homophobic and then failing to realize that there would be any subtext to killing off those characters. In a very, very rare turn for this kind of show there are two men with nude scenes and two women with nude scenes, allowing for some kind of equality. Historical dramas (and the quasi-historical Game of Thrones) are meant to have lots of naked women because it is the past and in the past naked prostitutes were everywhere. So even an attempt to include male nudity is a notable change. Of course the show can't go too far and so had both of its men be quite old and nudity fleeting, while its women are quite young and the camera lingers on them. Leonardo's friend implies he had sex with men. And in a way that's brave without actually being brave in any real way, Leonardo makes a comment that he might enjoy looking at men and then proceeds to have an extended sex scene with a woman. It all adds up to the show trying very hard to announce it's lack of homophobia while still delivering the heterosexual male gaze as much as possible. Game of Thrones's near constant heterosexual male gaze is consistently its worst feature, transposing that format onto a historical figure who is notoriously gay is just a cowardly decision.
Of course had they made Leonardo gay, it would have probably made for a very excited and evangelical fan base. All the people who were very, very loud about the "straight-washing" of Leonardo would have probably excitedly watched at least a few episodes. The show's star, Tom Riley, even went on weird apology tour of gay news outlets in order to try to convince people to watch the show (he somehow managed to come off both as sincere and also speaking the kind of corpo-huminoid lines written by PR people). Of course it would be a gamble that Starz's baseline audience wouldn't immediately change the channel in rejection of a gay lead character.
There's a lot of other stuff going on in the pilot, but it isn't particularly interesting. There is a secret society conspiring across time (The Da Vinci Code), a lot of Italian politics (The Borgias), and some ahistorical inventions (pretty much every dramatization of Leonardo Da Vinci). The only really interesting dimension to the show is the sexual politics, and even then it is mostly the metatextual sexual politics of the behind the scenes show. Everything else is a retread of other properties.
