Perhaps that level of sophistication was not available when they started out their FBW design. When it was - they already had so many machines in service, and pilots used to their side stick, that it would have caused more problems to uprate? Gosh they might even have had carriers specifying that the change must not compel any SIM time by force of financial penalty...
Seriously, if they had a working system making a fundamental change would be diffcult. Might be good for the longer term but Bus were still climbing the market at the time and such major changes could have been tricky.