What is the reason of using Pedersen Commitment scheme over HMAC?
In this case, there probably is no difference.

The Pedersen Commitment scheme is often used in cryptographic protocols because: 


It allows zero-knowledge proof to prove some properties of the committed value. 
It is perfectly hiding, which can be important in proving the security of the protocol.


Hash based commitment schemes do not have the above two properties. However, they may be more efficient and are preferable in certain scenarios.