It's to prevent teams from sign and trade deals that could potentially artificially decrease the market for a player.
Somebody

got too creative with the edges of the rules many years ago, thus we agreed to lock RFA's until the end of summer to make teams commit to players they bid on, and not make deals to flip players with say the team they're in bidding battle with, in exchange for compensation.
That said, I mentioned to Jim today, maybe we should discuss reducing the limitations a tad, and perhaps allow teams to trade RFA's to any team that wasn't involved in bidding for said player.
Something to contemplate and vote on for 2020.
#yesI'mguiltyforbeingcreativemanyyearsago!