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

I've been looking at 8b/10b lately so I'll take a crack at this.

My guess is that it's because K28.5 contains the extremes of 8b/10b encoding. More specifically, it has both a run length of 5 and 1 within the same word. In the case of "0101111100", if the receiver can correctly receive five 1s and a single 1, that is as stressful as a single word can be. There are other combinations that could do this but since K.28 has "111100", it's a little bit more limited.

It's only a guess but plausible I think.