Those first few posts just about nailed it. Notice Corbon's 110gr. JHP in .357. 1350fps from a 4" barrel. Their 115gr. JHP in 9mm+P is 1300fps from a Glock 17. They are almost identical when you factor in difference in bullet weight and therefore it wouldn't surprise me if the load was almost identical, ie, a fast burning powder.
Looking at my loading data for Viht in Loadbooks, .357 mag fired from a 7" bbl., the data for the 3N37 powder, a popular 9mm powder and my favorite 9mm powder and fast burner (very clean, low flash) is 1646fps for a 110gr. bullet at 35,000psi, and seems to be near equal to 9mm data for the same bullet weight given the barrel length (consider also I get 1500fps from a G17L with 115gr. +P and +P+, and 1600 with 110gr. from a 6" 686+). But things change when you move to the slower burning N110. The data from a 7" bbl, max load 110gr. XTP is 1909fps! That would still be smoking with a 4" bbl.
The 145gr. lead bullet goes 1368fps max using 3N37, so about 1000 or so in a 4" and same with the 147gr. 9mm in a G17. But with the N110, 1591fps.
The 125gr. XTP goes 1772fps max using N110, and I've clocked 1700fps from a 6" 686+. No 3N37 data.
Problem lies in that the slower burning powders, the N110 for example, use 18.4 grains for the 125gr. XTP! 5.56 uses about 20gr. of powder and it almost fills the case, keep that volume in mind, so the slower burning powders in the .357 simply fill the case up, and the 9mm case just isn't that big. It might accomodate 10gr. IF you compressed it, but that just isn't enough. The slower burning powders, they often don't burn all the way and have a huge muzzle flash, and may (the following is my opinion) not be able to generate the proper force necessary to operate a traditional size slide on an auto, with the DE being the exception but it is gas operated and not traditional sized. And I'd think you'd need a fixed barrel.
This about sums it up. Ultimately has to do with case capacity and slower burning powders. It would also be interesting to note that the .357 using 3N37 uses a few extra grains vs. 9mm to obtain similar velocities due to the fact it leaks between the barrel and cylinder.