Why are the Lucas generators so bad?
"Why" is more political than technical.
England failed to maintain their manufacturing effectiveness, much like their grip on empire. The combination of heavy-labor-Union dominance and increasing government ownership of the factories produced a perfect storm of incompetence.
Lucas, who actually had, and has, a fair amount of competence got caught in the middle. Instead of pursuing technical brilliance (like Bosch, for example), they were instructed to produce large quantities of the cheapest possible components. Thus, every element in, for instance, a generator for a small, low price car, would have been chosen for its cost rather than MTBF.
In the end, their generators, starters,alternators, switches, etc would exhibit a wide range of failure modes. Field coils, brush springs, bushings, regulators all "let the smoke out" in random frequency.
I don't really fault Lucas. Had they refused to make crap, the labor government in that era would have simply replaced the directors and managers to ensure the maximum number of jobs.
I suppose one could build up a Lucas generator that would run a long time, but it would require fabricating one-off components at great expense, rather than just replacing failed components with new, but inherently flawed, Lucas parts.
Of course, a great many do run for a long time. It's just that the failure curve is pretty fat.