why does a Bar of soap lose its lather toward the end?

Answer 1

Perhaps it only seems that way because the bar is smaller and you need to lather longer to get as much.

Here's a tip to get the most out of your bar:

When it's getting thin, get a new bar of soap and lather both up (as best as you can) and press them both together. Set them in the tray and let them dry together. They'll actually fuse and become one big bar of soap.

Answer 2

It doesn't. As the bar gets smaller it has less surface area to extract the soap from leaving you with the impression that it just isn't the same.