WAP-7's USP is not top speed but excellent acceleration. Since passenger trains have a lot more stops than Mail/Exp/SF, they stop frequently. A lot of time can be saved if the loco can fast accelerate.
On the same logic, 12951/52 Mumbai Rajdhani Gets a WAP5, while 12953/54 AK Rajdhani mostly gets a WAP7... All of us know that how large no. of halts AK Rajdhani has....