Each character required 8 bytes to define its bitmap. If all you want to do is define only the 52 uppercase and lowercase letters (which was allowed), that's still over 400 bytes. The entire computer only had 16,384 bytes of RAM, and not all of that was available to the user.

Also, there is the small matter of how you're going to load it. The obvious answer is to write a BASIC program to issue the font commands, and store the BASIC program to cassette tape.

Then every time you turn on the computer, you have to make sure the cassette is loaded into the tape drive, make sure it is rewound to the correct spot, type the command to begin reading a program, press play on the cassette deck, wait what might literally be a minute for it to read, stop the cassette, run the program, then delete the program from memory. This whole process is likely to take five minutes.

So you end up asking yourself, can I stand to just look at these ugly characters? And the answer is basically yes.

EDIT: I should add, the word processor was probably on a ROM cartridge. presumably they could have included a font on that cartridge and loaded it without all this tedium if they wanted a font to be available just for that application. So maybe not as bad as I am portraying it.