Because you're only using 2500kbps for a 1080p60 encoding. It's a very low bitrate for that resolution and framerate.
If you're shooting for 1080p60 on twitch, especially with anything fast paced, then you should be aiming for 6000kbps, and then go for a slower cpu preset if you need more quality after that. Otherwise, drop your resolution down to something like 900p or 720p.