Approaching a master's in humanities after STEM-focused undergrad
There is no reason not to try to do this and many reasons why you should. But the only valid advice will come from an admissions committee or a professor in the field who will look at your qualifications and goals. 

Be sure to present yourself in the most positive light relative to the new field, of course. 

There may even be advantages of your STEM background. Mathematics and CS, for example have applications in the humanities and the background can lead to many fruitful collaborations if you speak the language of both fields. 

But until you apply somewhere you have no idea of the possible outcome.