Not to point out the obvious or anything like that, but if you dismantle the two lenses you'll find that they're not quite the same. The 50/1.4 contains more glass and is a more complex design. It cost more to make that the 55/1.8. Going from f/1.8 to f/1.4 is not a tiny step.
Originally Posted by mhv
The 55/1.8 is an older design, dates from the days when "normal" lenses for 35 mm SLRs weren't at all retrofocus and had to be longer than the conventionally accepted normal focal length for the format (50 mm) to clear the mirror.
As to why people were willing to pay a premium for 50 mm and f/1.4 over 55 mm and f/1.8, I'm not sure. When I bought my first Nikon in 1970, I chose the 50/1.4 rather than the 50/2.0 because I thought I'd need the speed. I mean, it was often pretty dark where I then lived, even at noon. And I used the speed; I have a couple of barely acceptable shots taken a little after sunset that would have been impossible with the slower lens. But after I got a 55/3.5 and realized the absurdity of carrying two lenses of about the same focal length, I learned that a fast lens wasn't the only way of coping with, um, darkness and got rid of the 50/1.4.
The short answer to your question is, "Because."