Here are some tweets made by Kouki Yoshimune recently, explaining why they are making an anime out of Total Eclipse instead of the core Muv-Luv story:
I would love to have started with Muv-Luv, but the people who fund these things avoid shows with risk factors, regardless of the quality of the content. The truth is that we had gotten an offer to do Muv-Luv and Alternative from a certain large company during the planning stage, but even after 8 years we couldn’t come to terms.
To be specific, these risk factors include “It’s too long”, “It has a lot of nitpicky fans”, “It has (or appears to have) political/ideological elements”, “It’s based on an eroge”, “It’s old”, “Robots take too much energy to animate”, “SF doesn’t sell well”, “It’s not moe”. People tend to avoid these kinds of risks.
In that sense, the threshold for Total Eclipse is lower than for Muv-Luv and Alternative (at least, in terms of the different Muv-Luv works at the time the anime project began). And in fact, the good sales of the figures and A3 products helped reassure the minds of the people who fund these things.
But even so, I don’t mean to suggest that Total Eclipse is merely an underdog or stepstool (laughs). This is all kind of inside baseball, but the lives and futures of many people depend on this, so it’s not something that they do lightly or without thought.
I suspect everybody is assuming that we will simply copy the novels directly into animation, but I don’t like copying things exactly. The director also has experience making numerous shows, so we are taking care that Total Eclipse is a completely standalone work.
Although we’re covering the same major themes, what we’re going for is to keep the character development the same, while liberally changing the background setting and flow of the story. The characteristics of each medium are different, and since I myself am from the previous generation, I’m used to the story and overall feel, and even the characters’ faces, being different in each medium (laughs).
As the Total Eclipse anime’s executive producer, my job is to supply the director with as many choices as possible. The anime is ultimately the director’s film, and when it comes to making the hard choices, it should have the unity of a single person’s vision.