So I literally just now (okay, literally now about a minute and a half ago
) read this in an article on The Athletic: "For one thing, teams have become hyper-protective of their most hyped prospects."
It seems from other threads here that you are not alone in this perception of OOTP20 trading.
I can't say for sure that it is a universally correct perception, but if it is that might have something to do with the OOTP developers adjusting to the current realities of Major League Baseball.
Plus, for my money, anything that makes it harder to win consistently in OOTP (and therefore makes the game more realistic, IMO), is a good thing.