I won't tell you that there are tons of posts on this subject if you care to search.
In simple terms, and I'm sure many of the astute folks on this site can tell you in technical terms, when you use cans to mix, cans are a different environment, and what you hear in the cans may be wonderful BUT IT IS NOT WHAT YOU WILL GET USUALLY WHEN YOU MIX and then play back through speakers or monitors.
If you have decent monitors, you can set the bass, vocals, drums, etc., exactly the way you want them and also mess with EQ and dynamics. Then, what you hear on the mix when you are ready to do a CD or tape, is what you should get when you finish and play it back.
Tracks like drums and bass especially will not sound like they sound in cans, as well as vocals.