The Marvel Cinematic Universe is a vast, interconnected web of over fifteen movies, so it's somewhat inevitable that certain characters and plotlines will get lost in the mix.
With so much going on, some things just get left by the wayside, and as audiences respond to each new film, other things are changed or added. As a result, the universe's stories are constantly shifting, and plenty of threads are left behind along the way.
Just look at how Pepper Potts disappeared between Iron Man 3 and Spider-Man: Homecoming - despite being a huge part of the Iron Man trilogy and The Avengers - or how Thor has been moved away from his Earth-based escapades.
It's clear that the MCU isn't entirely set in stone, and something that works well in one movie could be completely absent from the next.
And while the issue of Pepper's absence was addressed with a few snippets of dialogue here and there (and her eventual return), there are certain plotlines the universe has abandoned - whether to pick them up again later on, or simply as an attempt to bury them completely.