In HyperLedger Fabric why can't we use the same PKI for Peer Identity and as well as TLS !?
Technically, you can use the same key/cert pair for both signing and TLS (the X509 cert will need to be issued with all of the usage and extended usage attributes required for both cases).
This, however, is not a recommended security practice. Best practice is to follow a "separation of concerns" when it comes to issuing the crypto material / identities. So for our samples we chose to adopt these best practices.
An example might be that you want to use one of the major 3rd party CAs (Symantec, DigiCert, Certicom, etc) to issue your TLS certificates but then use your own CA to issue the enrollment material.