Furrow irrigation( same reason most are mounted planters) and tractors being used as haying tractors is probably the biggest reason, would hate to use a 60" tractor on some of our sidehills pulling a baler. 
I know some guys that have 16 row 36" stuff, but they run an 8 row head, most are 12 row and under.
I know they make 12 36 heads, but that would be too big in this part of the world. 

Personally, if population and fertility are the same, here they will run right together, the yield bump comes when you can push populations a little higher and have the same plant spacing.

We thought about switching once but a very good agronomist who sees alot of both widths told us if you have a good line of 36" stuff youll never pay for the switch on yield bump alone "here". Were starting to dabble in cover crops and especially early innerseeding, the 36" will shine compared to the 30s for that purpose.