The filler episodes were traditional whenever a popular manga for boys was forced to put way too many episodes for economic reasons. While Naruto is the poster

The filler episodes were traditional whenever a popular manga for boys was forced to put way too many episodes for economic reasons. While Naruto is the poster child of the problem, as the ratio of anime only content vs adapted is through the roof, we could see it many years before with Dragonball. Animation standards have gone up enough, and there are so many more competitor series, that just needing to fill a broadcast TV slot forever just isn't as important as back in the day. The levels of animation repetition of the Sailor Moons and Captain Tsubasas of the day, which allowed for a lot of episodes with a reasonable budget, would be seen as unacceptable today.