I think there are a few things going on.
First, I think risk management plays a role. If you're defending 350 in an ODI, you can risk getting hit for six to get a wicket. If you're defending 260ish, the risk doesn't seem worth it. That's why we see Adil Rashid thrive in run-fests in England, while Yuzi, Kuldeep, Tahir, Shadab etc bowl quicker in the air in conditions where 280 is a good score. Seems counter intuitive to bowl flatter in low-scoring conditions, but it makes sense if you think about it.
On top of that, when you have a slow-paced pitch, it is better for a spinner to bowl quick and flat. The ball won't come on to the bat anyways, and by giving it air you're giving the batsman a chance to use his feet to generate power. It is similar in a way to how quick bowlers resort to bowling cutters and slower balls in these conditions. All you're trying to do as a bowling attack is hit that middle ground of not having pace on the ball, but also not giving batsmen something they can use their feet against. A lot of ODI and T20 cricket is being played in conditions like these, so we're seeing more of these bowlers.
Lastly, with regards to Jadhav - he's not your traditional flattish dart flinger. What's been making him effective is slinging the ball in, and how little spin he actually puts on the ball. The amount of spin on the ball is important for a batsman to generate power behind shots - the more revs on the ball, the better it flies off the bat. So when Jadhav is slinging in short balls with no spin that only rise up to shin height, he doesn't just make it difficult for the batsman to use his feet to generate power, but he also takes a lot of energy off the ball.
In a way what Jadhav is doing is kinda like the knuckle ball in baseball. And it isn't new really - the slow non-spinner has always been weirdly effective at lower levels of limited overs cricket. The reason they succeed is that they require pretty good technique and timing to put away, and often those just below the Elite level don't have what it takes. I think it's doing so well in modern ODI cricket because batsman now are more reliant on power than they ever have been. Techniques just aren't as good. Plus scores are higher now than they ever were - you can't get away by milking the fifth bowler for 4-5 runs an over like you used to. As a result, a lot of modern batsmen are trying to hit Jadhav out of the attack, and end up losing their wicket in the process.
I don't know if Jadhav is just a flash-in-the-pan that will be figured out sooner than later, or if he is really heralding a new era in spin bowling. But it would be pretty amusing if sincere pie chucking becomes a thing. It would be like how dibbly dobbly medium pace ruled ODI cricket for a good decade or so.
