There are of course the obvious answers -- Deadpool is an R-rated character, and Hollywood is nervous about making R-rated superhero movies, and Black Widow is a female character, and Hollywood loves ignoring the fact that people watch films starring women. But there's actually a much simpler deciding factor at work here: whether the character wears a mask.
Check this out: Robert Downey Jr. gets paid all of the money in the universe to play Tony Stark, so much so that his salary for The Avengers broke box office records and forced mathematicians to invent a new decimal point. But any time you see Iron Man on screen, that's not Robert Downey Jr. That's either a digital effect or some stuntman in a costume getting paid less than the cost of the tires on RDJ's trailer. The same thing is true for Spider-Man, the Hulk, and any other character that wears a mask or is a CGI dinosaur Hercules.
It allows Marvel Studios to get away with paying the lead actor substantially less money, because the actor isn't actually on screen all that much. That's why the amount of time Robert Downey Jr. spends inside an Iron Man costume has gotten progressively shorter from film to film as his asking salary has increased, and why three different actors have played Bruce Banner in three different movies ... freaking anyone can be the Hulk.
"Mark was great in the role, but we can't wait to show you what Cheaper Late 30s/Vaguely Smart Looking/Mildly Broody Actor #4 does with the character!"