(This is assuming 144hz 1440p isnt being displayed period, so its not a tf2 issue)
What cable are you using? Because if you're using HDMI and didn't change the cable that you were using for your old monitor, its possible that the cable is version 1.4 or older, which supports 144Hz on 1080p but not on 1440p.
If you're using DisplayPort instead, the same situation could be happening if the cable is version 1.1 or older.
Your graphics card shouldn't be a problem since it supports up to DP 1.42 and HDMI 2.0b