a 'cause and effect' is a direct result of something from something else; What was the cause of the delay? A tree had fallen down on the road. We're talking about the reaction to something, maybe without intention.
reason is more normal (an explanation - why??)
so, your sentence should be 'I couldn't improve … because there weren't many resources/as there weren't many resources', etc Or, 'The reason I couldn't improve.. was that there weren't..'
If someone asks you 'why?', that's your reason.. true or not!
'cause' is not appropriate because it didn't REALLY stop you from improving!