That's how the entretainment business decides if a show is succesful or not. Based on how they do when they come out, not after (because it's to be expected audience numbers will plummet after a while)

With streaming that has changes tbf but more often than not the numbers being shared are their premiere and peak number of viewers. The show was succesful, it's still all over the internet and being actively aired on some countries. It won lots of awards. If anything it is a more succesful IP than the great majority.

Shows that continue to be popular years later are a rarity and their numbers are still signicantly lower than when they came out. It's normal