Why is there still no 64-bit client for Steam?

This explains why.
http://www.howtogeek.com/194119/why-are-most-programs-still-32-bit-on-a-64-bit-version-of-windows/

Basically, a 64-bit software is only beneficial it uses more than 2GB of RAM. Steam client itself does not need that kind of resources, but to develop and maintain both 32 and 64 bit software takes time and effort, not to mention Valve needed to support Mac and Linux as well. That's why there is no need for 64-bit.

TLDR:
Because there is no need for 64-bit client.

Until someone comes up with a very cogent argument for 64b Steam, I'd stick with my assumption it's not going to make any significant difference except using slightly more memory.

Because they have to count all the money they made from us?