Probably had nothing to do with the widespread corruption among executives, union bosses, suppliers (remember the gasket contractor who was supplying BMC/Jaguar with "cork" gaskets that were actually sawdust?), and dealers.
Roll most of a nation's automotive industry into one big company and it's bound to fail. Heck, roll it into three companies, and they are all bound to fail on a long enough timeline -- just look at Ford, GM and Chrysler. Has a lot to do about inbreeding of leaders within the industry.