Because Jolts are simple, cheap to produce and powerful enough for kids not to care about it. Hasbro wants to get as much money as they can while also keeping as much money as they can. And not R&Ding stuff is a great way to save some cash. And they already have a reliable design, so... Why bother?
Everything the experts said above, Memes 👍 It all comes down to money. The Jolt is simple, inexpensive to produce, cheap to ship, takes little shelf space, and the price point appeals to both Nerfers, and more importantly parents.
As PedroH said, the product development is already done, and there are just two moving parts; the plunger and the spring. There's a giant injection molder in China programmed to crank out a thousand Jolt shells an hour, day and night. Just type in a color.
Bottom line, the profit margin as a percentage is probably better on the Jolt than any other springer, and most certainly any other flywheel blaster.