Nana is the main girl, read the frickin' title you guys, obviously she's gonna have content later than everyone; you don't have dessert first. If Moonbox rushes sex with Nana, it'll feel cheap, rushed and won't feel as genuine (as genuine as an Incest relationship can be). Gotta save the best for last. If the dev gives us Nana on a platter with her legs spread open in the first episode itself, how is he gonna make any money?
Have sex with the main girls too fast and people complain about bad pacing and a bad story; take your time with building the relationship and sexual anticipation properly, and people are impatient. You can never win, with some people.
You'd think people would learn by now, from seeing almost every Incest VN give us the main girls (family members) later than everyone else. It takes time to build up to having sex with someone who you don't know; when it comes to a family member it would take even longer, because you have such a huge moral dilemma to overcome, and other factors to consider as well.
I love the Incest genre of VNs, but I'm ashamed to be associated with you impatient lot. Ignorance doesn't have a limit, on the internet.