Carl:

Here's my theory and it's only a theory.  The irons built today feature bounce design and sole design that launch the ball higher, all things being equal.  As the ball launches higher, irons need slightly stronger lofts to produce flatter flight with less spin. 

Ryan Crysler
Head Instructor
My Golf Performance Center
512.850.5721
mygolfperformance.com

Ryan is likely correct from a technical perspective - I don't know and that seems logical. I hypotesize how much is attributable to certain club manufacturers having stronger lofts, such that the average player can tell his buddies how far he hits his 7-iron wiht brand X; therby enticing his buddy to play that particular club. There is no consistency in specs on lofts between manufacturers - they are often close, but a few degrees can mean a half-club difference or more sometimes. Just a thought.

I don't know about stock irons but I delofted my irons by 2 degrees. I definitely agree with Carl about the new irons launching higher, especially the forgiving game improvement irons. I'm currently hitting the MP58's and the club fitter still said that the ball flight was considered high.

I also think that it adds some shots in the bag where a lower trajectory with a shorter club is welcomed...

But a disadvantage with delofting clubs is that you are effectively decreasing the bounce on your irons. 

Carl:

The biggest influence is the pros vs every day duffer.  We watch pros hit 9 irons 170 yards, 5 irons 220 yards, so what is the result, we all want to hit that shot.  The club manufacturer want to sell product so they deloft clubs to give you the distance.  True some of the added distance come from better shafts and more responsive club heads.

Years ago (and not to many) a PW was 49 degree now depending on brand they vary from 46 down to 44.  the PW is now equivalent to a old 9 iron.

I guess this change is being stimulated by ego and of course bragging rights.