Why does CFR §91.215 part (b) list specific areas for transponder operations?
I think what you're missing is that not all aircraft have transponders. The key words from 91.215(c) are (emphasis mine):

[...] each person operating an aircraft equipped with an operable ATC
transponder maintained [...]

To put it another way, (b) says &quot;in order to fly in these areas you must have a transponder&quot;. Whereas (c) says &quot;if you have a transponder, this is when it needs to be on&quot;.