There's a supercut of all sex and nudity scenes in the series which is near as long as an episode. This is what you call a hook and baiting. Gratuitous violence and character deaths and even interesting lore isn't enough and it work for HBO to attract loyalty. Sex sells. It is known. Valar Morghulis.
If it come to it, FOX could take on it with HBO-like fund but they love cancellation so much especially if the pilot didn't attract more than 4-5 millions viewers. SyFy have The Expanse which already is their ambitious project (and they barely adapt half of the first book). The most realistic was BBC America especially since Bioware is Canadian and they support Doctor Who productions which could use more space-worthy stories.
Well if you're considering all the episodes up to season 5, that's like a 1: 50 ratio which seems rather low for the emphasis you've given it. The violence is also rarely gratuitous and most always fits the situation and the setting. Saying that people see game of thrones for just the nudity would be as misleading as saying people buy ice cream Sundays for the cherry on top where the nudity is the metaphorical cherry. And how did you get to Valar Morghulis from sex sells?
As for a mass effect series, I'm not an advocate for a tv series for a number of reasons. The first being that it needs a lot of CGI or some really good animatronics to make believable aliens, given that we interact with aliens quite a lot in the ME universe. A series might not have the budget available for such a level of investment over a long time such as the running period of the series. I just don't wanna see a mass effect consisting of mostly humans or the occasional cheap cgi aliens.
Another reason is that Mass Effect has mostly been about the characters, the setting and the back lore to me. Shepard is well and good but everyone has a different way of playing their Shepard and trying to make a story about Shepard will invariably clash with the views and headcanon of a lot of the game's fan base. The story is also to be quite honest, not really all that sophisticated or deep. A different story in the same setting has much more potential for a more in depth and subtle narrative that I feel has more potential in a movie/series adaptation.
A movie in comparison to a series is shorter and has more budget and production houses behind it. And since it's a single work the developers can also spend more resources and effort than for a series where it might be spread over the much longer course of production and episodes. This part is my speculation of course but then again, I can't remember seeing any series with lots of CGI that was done well and consistently used throughout the series. The average movie also has much better actors and directors than the average tv series as far as I know.