Why weren't more characters from the Marvel universe featured

[TMC]

I don't think that until the first two reunion movies that aired on NBC from 1988-89 (that featured Thor and Daredevil respectively), any of the characters from the Marvel universe appeared in this iteration of the Incredible Hulk. Was it simply because the producers were reluctant to incorporate too many fantastical elements (which ironically, is what a character like Thor pretty much is) and wanted to keep things as "grounded in reality" as possible for a comic book show? Was it because at the time, it was more of a hassle for TV and film producers to obtain the the individual rights to the characters? Keep in mind that this was long before the MCU and Disney owning Marvel, which meant that they had to lease each their characters to film studios as oppose to doing everything "in-house".