Time, mostly. 5s are only released so often, 5s come out to tie-in to other media, there are loads of characters to get through which, tied to a limited release rate and an outside source dictating who can come next, means it takes a while.
There are plenty of characters that we already have that could have a 5* version, Scarlet Witch, Juggernaut, Magneto, Doom, Iceman (616, not teen snowball face that we have now, we don’t have a proper Omega-level 616 Iceman yet), etc, and plenty plenty more that could be added (Adam Warlock, Namor, Red Skull, Super Skrull, Dormammu, Odin, etc), and plenty of MCU characters that could still get a 5* version (thankfully we’re down to most of the “second string” characters, Gamora/Rocket&Groot, War Machine, Falcon, Vision, Scarlet Witch herself, where SWitch probably ranks highest, but we could just as easily get a nano-tech Iron Man or a post-Ragnarok Thor or possibly a new Hulk or Cap for Endgame, not counting Captain Marvel, Spidey, and maybe Mysterio as candidates for other MCU movies next year)
I wouldn’t be surprised if she’s placed on the long-term calendar to come out with her show (followed up with a 4* Vision, maybe?). Fortunate in that the show makes it seem more likely, more “inevitable”, unfortunate in that we could have 6-10 5’s before it’s her turn, you know? And my quick brainstorming list above has a total of 23 on it, not counting any curveballs they might throw our way (comics are resurrecting a bunch of people, could get another Cyke or Professor X or Wolverine or Jean as a tie-in there). So once you start filling in some tie-in spots (one 5* for each of the three movies, for example), it starts limiting the number of spots for everyone else, which makes it harder to get everyone in.