Why K28.5 is more frequent to be used among the 12 control character in 8b/10b

I was referring to your initial question "why the control character K28.5 is more frequent to be used among the 12 control character". Now it seems that you are actually asking which properties of the character K28.5 are motivating it's usage, which is a different question.

As a user of 8B/10B, I normally take the protocol definition as a matter of fact. But some points can be said in this regard.

Firstly, there are three "Komma" characters in the 8B/10B code (characters that can be used for synchronization purposes because they can be unequivocally identified in a bitstream), two of them (K28.1 and K28.5) can be used repeatedly, so the initial choice is already reduced from 12 to 2. You can review some detail considerations about 8B/10B code design in the description part of the original IBM patent US4486739. Whether K28.5 is better suited than K28.1 to be used as idle character or if it's just a convention doesn't bother me that much, but may be you find some facts in the document.