Why still we are using classical key exchange rather than quantum idea Why still we are using Diffie-Hellman idea  rather than BB84 or B92 like 
quantum protocols for key exchange?
Well, not everybody is willing to put an optical line from each person to another or install the hardware components to do the quantum computing, for one. You cannot just send the information over the internet protocol or even electrical wiring.

QC key distribution is commonly used by large organizations and governments that want to distribute secret keys relatively often (otherwise you could just send it by courier).

Besides, without fully functional quantum computers for analysis, DH is still considered secure, at least when the proper authentication and verification procedures are in place.

Note that QC is no panacea either; you could for instance try and retrieve information from the equipment or find other holes in the way the QC is actually implemented. More info in section 4 &amp; 5 here.