Because that's what is actually supposed to go in it: the AW4 is not a Chrysler product, and the manufacturer has never specifed anything other than Dexron II or III. Chrysler stopped carrying D/M, which is a GM/Ford licensed product, the instant the AW4 was no longer under warranty. ATF+4 is much thinner than D/M and known to cause slipping in many transmissions. You can use it safely in your transfer case but that's about it.

 

Because many folks have EXPERIENCED slipping problems after using the ATF+4 on their older AW4s.

 

I don't think it's even a case of back-room eggheads thinking something "should" work. When the XJ fell out of warranty Chrysler suddenly dropped all third party manufacturer-specified lubricants and switched the XJ to the same oils as the rest of their vehicles (which have different requirements). Pretty sure it was a beancounter decision.