Kay Steiger and Scott Zeger talk about ways college professors could make introductory science classes less off-putting and encourage more people to major in STEM fields (science, technology, engineering, and mathematics). I think one has to take a deeper step back and ask whether STEM faculties actually want more people to major in their fields.
Different universities are structured differently, but what I recall from reporting on university finance when I was in college was the following dynamic. The STEM departments received large quantities of outside money to conduct research, and they used their pool of graduate students as laboratory labor. An increase in the number of undergraduate students the STEM departments had to teach was a drain on the supply of graduate students, since graduate students would need to be diverted from lab work to teaching assistant work. In the humanities, the situation was quite different. The humanities departments don't have lucrative outside research grants that require graduate student labor. Instead, their economic problem is a lack of demand for the labor of humanities Ph.D. students. An influx of undergraduates into the major prompted a disbursment of central university funds to employ more teaching assistants and lecturers thus helping to solve the problem of underemployed humanities Ph.D.s. Not coincidentally, the humanities faculties tended to be very eager to recruit people into majoring in their field and were eager to get nonmajors to take at least a few elective classes. The STEM faculties, by contrast, tended to ward people off with intro classes designed to scare people away and complicated webs of pre-requisites that aimed to weed out nonmajors.
Now up the road was MIT, which has a different institutional history and a different ethos and you got a different result. But most universities don't have that particular institutional history and ethos and my guess is that, intentionally or not, their faculties are responding to the same objective incentive framework. There's a lot of sense nowadays that we, as a society, want to push more students into these fields (I'm not sure that consensus is correct but leave it for another day), but if that's what we want to do we need to structure the financing of colleges to align with that goal.