From a business standpoint, Valve has an alleged 4-5 developers working on Team Fortress 2 as of 2019. Team Fortress 2 is a game that is close to thirteen years released, there's not very much more they can do, especially when Team Fortress 2 likely does not make as much revenue from them compared to Counter-Strike: Global Offensive and DOTA 2.
It's just the way it is for now, Team Fortress 2 isn't sustainable enough to hire more developers.