The original CAB1 came out in in the mid 90s. The serial output was designed to support expansion and was useful to monitor or send TMCC commands using a computer. The serial data rate was sufficient for TMCC which sends out 48-bit packets. IC Controls and ZStuff subsequently used the serial output as their device input. The serial I/O drivers are simpler (less code) than TCP/IP which makes a huge difference (cost, power, memory) when implementing the logic as firmware.
IIRC, networking with PCs in the early to mid 90s was, shall I say "interesting", until Windows 95 came out and provided a consistent network stack. I have a box of 28.8 modems from that era. Serial comms between two devices in that era was simple and reliable and serial ports were standard in PCs until about 10 years ago.
20 years later we have several orders of magnitude increases in network data speeds, but the data rate required for TMCC and Legacy communications remains the same. Most commands remain 48 bits (3 bytes). The extended Legacy protocol allows three 48-bit packets to be aggregated into a single command. When a 3-byte Legacy command get translated into WiFi packets, a 3-byte packet gets framed into a TCP/IP packet of 68 bytes plus all of the TCP/IP handshaking overhead.
Just because your PC doesn't have a serial port in the back doesn't mean the Legacy Base's reliance on serial comms to external devices is not modern enough. Get a LCS WiFi module if you want network connectivity, or use an inexpensive USB to serial adapter.