Why were NES cartridges unreliable? Why did they take so much effort to get playing properly?
AFAIK, it's not the fault of the cartridges themselves.

The original NES-001 had a zero-insertion-force scheme which traded off ease of insertion for (what was later discovered) reduced effective life of the contacts. The lack of scrubbing action combined with everyone 'blowing' on the edges of the cartridges (plus lower tolerance for bent pins due to the wide insertion angle) led to the redesign that you cited. I've replaced the ZIF socket inside an NES-001 and after that, it worked like new for the rest of the time I kept it, with virtually no bad contact issues.

I would speculate that since so many NES carts were blown on, they would show a higher propensity of oxidation which may lead to problems even with a straight-on connection. Also, since the system was on the market for so long, the number of 'old' cartridges out there would be high, and they would likely also have some oxidation issues just due to age. When I built up my Atari Flashback 2 system with a cartridge slot, I had to manually scrape many of my old cartridges with an X-acto knife to get the contacts clean enough to actually connect.

I can certainly recall occasional seating/contact issues with my Sega Genesis system after a number of years of usage, so your observation may not reflect the population.