When they re-signed Kreider to his deal, that was basically the end of Buchnevich in Manhattan. They don't have the cap space to commit to another winger when they already had Kreider, Panarin, Zibanejad, and Trouba under contract and then added Shesterkin and the Fox deal. Maybe if they actually had a cheap, cost-controlled center to rely on and could let Strome walk, but that's not something we have so we may be looking at another fat deal for him added on top.
They could have kept him this season, but would have then been faced with the prospect of losing him for nothing at the end of the season--assuming that they would have been in playoff contention and not wanted to move him at the deadline. So, they chose to move him before the season. The return was obviously not great, but I think the other GM's understood the situation the Rangers were in and used it to their advantage.
It just wasn't going to work and I think it was pretty much known for at least a year before Buchnevich was moved. Were you just not paying attention?