I mean, the most literal meaning of "a bias" is "a slant," which is where you get terms like "bias tape" and "bias cut." From this literal sense of "slant," yo

I mean, the most literal meaning of "a bias" is "a slant," which is where you get terms like "bias tape" and "bias cut." From this literal sense of "slant," you get the figurative sense of "inclination" or "preferential treatment." This meaning appears in quotations hundreds of years old. Finally, as "biased" stops being a fancy newspaper word and enters the popular lexicon, it gets subjected to regular pronunciation tendencies. And one of those tendencies is the simplification and elimination of -d at the ends of words. A lot of people in the US pronounce "hand me that" as "hammy that," "send me an email" as "simmy an email," and so on. So it's not surprising that saying and writing "bias" instead of "biased" is popular.