Nominal Never got why this game was hailed as one of the best side scrollers on NES. I wouldn't even put it in the top 20. It felt like some short promotional game you'd get for free after signing up for the Ducktles fan club or something. The Sneak King or Pepsi Man of side scrollers.
It wasn't BAD, just a really short and ridiculously easy thing you'd play for 20 minutes and forget about it. It wasn't that hard to blitz through the whole game without taking a single hit. I think I was 10 when I played it and even at the time I thought it felt like it was made for very little kids.
cognitivedissonance All mechanics worked in a seamless way, and were unique from any other platformer at the time. Scrooge’s pogo stick cane was an inspired, strange and, for the time, very unique mechanic. There were plenty of secrets to find, and had fantastic golden age Capcom music.
It’s a matter of perspective.
Mother Lumper Any NES game that looked good and was relatively easy was extremely popular. Kids didn't want to be made aware of their deficiencies, they wanted to steamroll through something and feel like they accomplished something. Same reason Mega Man 2 & 3 are the most common and beloved of the series. Super Mario 3 isn't hard. Contra has a 30 life code that everyone abused.
Spit Spingola It looks good and captures the spirit of the cartoon. Also when it came out, 1989, most other games based on popular movies and TV shows were horrible. Compare it to stuff like Back to the Future or Ghostbusters on the NES. As a kid you'd buy or rent those games and be horribly disappointed but Duck Tales kind of delivered.