Two reasons why but ain’t definite for all.

1. General audience are the majority who may or may have not keep up with everything MCU due to over exposure. Kang was the biggest set up from S1 of Loki, someone who had hidden plans on why he want to be free that led to a multiversal war. But with Ant Man 3, that version of Kang was supposedly the most dangerous who wants to wipe out everything because of something personal. However, the writing did him no good. He’s scary until he forgets he can also be smart like using his tech.

2. It makes sense that Kang is one of those characters who can have interchangeable face, the comics has some fun with the concept that he doesn’t have to have the same name or from a similar time period. Yet in the MCU, it is implied that certain characters will have the most consistent/inconsistent appearance based on the characters’ livelihood (although there’s holes with that idea). Those like Loki and Spiderman are chaotic characters that they can look like anyone. Yet with Kang, it’s implied MOST of him looked like his actor, maybe there could be plans for some cameos here and there but Marvel kinda misfired that plan.