I think the "why" you are looking for is this:
The 737 was designed a long time ago. To get rid of the FE some things had to be "automated". When you get a bleed or pack trip, you do not have to change any switch positions. If you go up and start moving switches, then they are assuming you are intending to do something, like wing anti ice.
Next Gen is designed to be flown as a common fleet with the old ones. That is why some of the things that could have been improved were not.
BTW there is a 727 hiding inside most Boeings. Just automated in some.