Why is 2 Channel better than multi-channel?

I hear that the music fidelity of a multi-channel AV Receiver/Integrated amp can never match the sounds produced by a 2 channel system. Can someone clearly explain why this is so?

I'm planning to upgrade my HT system to try and achieve the best of both worlds, I currently have a 3 channel amp driving my SL, SR, C and a 2 channel amp driving my L and R.
I have a Denon 3801 acting as my pre. Is there any Pre/Proc out there that can merge both worlds with out breaking my bank? Looking for recommendations on what my next logical steps should be? Thanks in advance.