Difference is that back then the hardware demands were less energy dependent which means they generated less heat. By that the lifespan of the system is much longer barring poor storage conditions or rough treatment. The more processing power, the more heat generated, the less viable the machine becomes without adequate cooling and storage. Also the cartridge system had few (if any, depending on the system) moving parts or machinery so hardware failure was also less likely. The standardization of CD/DVD/BluRay media and adding hard drives to consoles has limited the lifespan considerably. Same with fans. Every moving part in a console is one more thing to burn out or fail at some point.
I'd also go on record that of all the systems I've had, none of them have ever burnt out or failed due to design flaws or poor conditions or failed hardware. I keep my systems clean, cool and well ventilated. Only time I've ever lost anything was a Wii and 360 because of a direct lightning strike that overloaded the surge protector somehow. The TV surprised me though and survived that encounter with nature.
EDIT: Posted too early, forgot to add closing statement.
So in essence the older consoles weren't better made, just had more primitive designs which had less moving parts and heat generation leading to durability but without processing power and limited storage.