Why couldn't the CRJ100/200 be equipped with triple-string autoland?
I have experience on that program and can tell you that:
A:  While anything is possible, this would cost a fortune to implement, requiring a complete avionics overhaul.  For a single potential customer?  Snowball's chance in heck.  The customer would have had to commit to paying the development costs, many zillions, which would also have a snowball's chance in heck of happening.
B: By the mid 2000s, no development money was being allocated for that sort of product improvement on the 200. By then the program funding was generally only to deal with airworthiness issues and upgrades to deal with avionics obsolescence, as development money focused on the 700/900.
C: British Airways aside, there was no demand to speak of from the industry for that kind of capability, that anybody would be willing to pay for. Adding HUGs on the 700/900 to get Cat IIIa cert was more than sufficient.  On the 200, even to do that would have required an avionics suite upgrade that no regional operator would be willing to pay for (again, Snowball's chance in...).  Regional operators run on very thin margins.
In the US, by the late 2000s, Regionals would not even implement optional Service Bulletins unless there was a financial payback of ONE YEAR or less.  That meant that only SBs mandated by Airworthiness Directive were being incorporated.