8. The X-Men's Neutrality
The X-Men are one of the most powerful groups in the Marvel Universe, and if anyone knows anything about facing hostile public sentiment it's the mutants. Since their publishing debut in 1963, the X-Men have always served as an allegory for civil rights, and they would normally have fit right in during the struggle over the SHRA. Unfortunately at the time the mutants were dealing with much bigger problems, mainly the after-effects of M-Day, which saw the decimation of nearly the entire mutant population, leaving only the "198" to continue the race. In Civil War #3, Iron Man reaches out to Emma Frost (whom he apparently had a romantic connection with at some point) about the X-Men joining the pro-registration movement. Emma refuses, stating that they know what it's like to be persecuted while fighting the good fight, but reassures Stark that if left alone they will remain out of the fight and not assist Captain America, either. Due to the fact that the X-Men film rights do not currently belong to Marvel, none of that will be able to be explored, but it does give Marvel a convenient reason why they're not included.