You're probably right to a certain degree. It's possible those Regs were designed around a time when it may not have been possible to have an Electric AH that was reliable enough, perhaps these days it may even be possible to get an exemption to replace your T+B with an Electronic AH but I can't really see the reasoning, being an electrically driven mechanical device that would most likely have to be permanently activated with the aircraft electronics, in the event of an alternator failure, would this be an unacceptable extra drain on the battery compared to a T+B?
The other reason is that the T+B would be, IMHO, superior to the AH in the event of your Primary AH becoming U/S for the exact reason that others have mentioned that you can execute a pretty damned perfect 180 with just the T+B and your Watch which is not as easy to do with the AH. If you're in the soup and the AH goes then presumably your DG is also going to be gone as well and trying to use the damned Mag Compass when underpressure and with it bouncing around does not sound as appealing to me as starting a turn on the top of the minute then rolling out when it gets back to the top of my watch!
A T+B also has the upside of being able to keep your aircraft in balance which an AH doesn't do which is for passenger comfort and generally speaking I've had enough AHs start to go a bit wonky on me but I can't say I have ever seen a T+B that wasn't dead on accurate.
In a twin engine as well, in the event of an engine failure, it is far easier to just step on the damned ball and get it centered to correct for the yaw.
You'll also find that just about every aircraft has one installed, infact I've personally never flown an aircraft that didn't, so why bother to rewrite regs just for the option of removing something that everyone already has, I can't see why you would remove it and bringing back to my earlier point I've never seen one fail so I'd presume they last a really really long time and hardly ever require replacement so if it ain't broken, don't fix it!
Same goes for the regulations, if there was something better out there to replace it with and the industry was clamouring for it then they might look at it, but as the point has probably never been raised and there is apparently no problem to fix, why fix/fiddle with a regulation that isn't broken?