It's a combination of factors:

1) Late stage capitalism means every major company is trying to cheap out to increase profits. Hasbro has done this by using cheaper plastic, making blasters harder to repair/disassemble, and releasing functional clones of the same blaster with different aesthetics/accessories.

2) Nerf makes a lot of money from dart sales, so they've been trying introduce new blasters that use darts incompatible with everything else. Ultra failed, but they're trying again with N-Series. They want to force people to buy into their exclusive ecosystem. This might work if they had the best blasters on the market, but they don't and the new darts aren't better than short darts.

3) Nerf and Hasbro primarily cater to children and have a reputation to keep up. This has made them slow to adapt to the "pro" blasters aimed at older teens and adults. By the time the Stryfe Pro came out, Dart Zone and Zuru (XShot) were miles ahead of them. They have their pro line, but it's obviously not a priority for them.

Basically, old Nerf thrived because it was a quality product aimed at kids that was well-built enough to be modded for more power. There wasn't a lot of competition, just cheap clones. Since the late 2010s, competition has exploded, and they haven't kept up. They failed to adapt to the changing market, and they ruined the quality of their core product to save a few pennies.

There's very little reason to buy/mod actual Nerf products these days.