The answer is : because the writers wanted it.
The same way Trip did not recognize his father or Marty did not recognize his partner.
I know, it just make the people look slightly silly
and at some points in. The Lich part 1 , where Cape and Marty, are looking around squabling at each other, there is a few moments where i think Marty might be close to figure it out, or maybe just thinking it, could be the slightly spitefull voice Vince/Cape used, or something, I am not sure, just thinking