Why wasn't the networking in doom just done as TCP/ip (or UDP/IP)?
In the later 90s IP pushed it out, partly due to Novel Netware fading away, and partly because IPX didn't scale to large networks the way IP can.
Googled tsr, terminate and stay resident if anyone else is reading this.
There's a decent interview with him that relays the story: https://episodiccontentmag.com/2016/08/26/kali1/
Because "internet" barely existed at the time doom got out. People played multiplayer games mostly in the same physical room.