Because for a long time, on most computers, the telnet client was the closest thing to an "open a tcp socket to this ip/port and connect the i/o from it to stdin/stdout" application you can get without installing something or coding it up yourself.

These days we have netcat/socat and others, but they're not reliably installed, while telnet used to be generally available because telnetting to another machine was more common.

These days, the answer would be to use a netcat variant. In the past, telnet was the best we could be confident would be there.

In corporate environments, netcat was often banned as it was seen as a "hacking" tool. Having it installed would sometimes get the attention of the security folks, depending how tightly they controlled things.

You don't even need netcat or socat for that, probing /dev/tcp/<host>/<port> from the shell is enough.