Simple do you want to get possibly hit by a jab or a cross? I'll take eating a jab over walking into a cross any day
First off, who says a jab and a cross are the only punches you can throw?
Second, don't always assume the power hand is the rear hand....... just because it is the hand in back doesn't necessarily make it the "power hand", go watch a couple Mike Tyson, Oscar De La Hoya or Joe Frasier fights........ would ya wanna walk into those left hooks?
It's not an absolute rule, but you assume a lot of risk when you start moving towards that thing. Most people are not comfortable taking that risk.
You assume risk regardless......... there are a lot of variables that can come into play, reach, height, stance, style, etc..... and it wouldn't be hard to write several pages on this topic......
But let me give you this scenario. Both orthodox fighters using the most basic fundamentals, your circling to the guys left and he throws a jab, you slip the jab outside, which way does your head move after you have slipped that jab before you can keep circling to your right? Yep, it moves back to your left in the center of your stance........ What do ya think guys who want to land a right hand throw behind that jab? Yep again, a straight right hand, that your head is now moving right back into!
Alternatively, your circling right toward his power hand and you slip his right hand to the outside and keep circling right........ Recovering from a missed right hand takes a little longer and requires more effort, hence giving you more time to counter or create offense. In short, if you make him miss a jab you still got to worry about his right hand, if you make him miss his right or power hand...... well that is probably less risky scenario to create offense, not more risky..... just a thought