They kept the car basic because the SRT team was basically making a point, they managed to pack a LOT of HP into a reliable engine but cut a lot of comfort features of the car to save money, you can push almost 400hp on that engine without replacing stock internals, if you look at the original advertising for the car it was all about the speed, they didn't care about much else, 03's didn't even have LSD, they had to tack that on as standard for 04 and 05 because its hard to have that much power without it.
The MS3 is definetly targeted more at giving the FEEL of SRT power with incredible refinement, they push the torque at lower RPM's because that makes a car feel powerful, honestly, stock for stock MS3 would probably lose by half a car length on the 1/4 assuming equally skilled drivers but most SRTs on the road today will take a MS3 in a straight line because of mods, on a road course I very much doubt the MS3 would lose even to a moderately modded SRT, ACR editions might be able to hang because they could corner better but the MS3 is still better in turns.
I am one of the rare people who actually likes the CSRT, in fact if I could have gotten as good a deal as I did on the MS3 I would be on a different forum today but it does have far too high a center of gravity so the car ends up with the same issues as the NSRT in that it can be modded well and is good in a straight line but its not so hot in corners.
Right now there are 2 issues the MS3 has to overcome and I can bet that once those are fixed or worked around the MS3 will have little trouble being modded the way the SRT is, first is getting rid of computer controlled limitations of boost, airflow and throwing codes with heavy mods and second is figuring out what kills the engine (though it really seems to be the throttle plate closing too early), once those are figured out the aftermarket will explode for this car.