Risa

I suspect that it had to do with the WGA contract at the time that TNG was in production.

Writers with a script credit got specific residuals for characters they created in a script according to what Beyer wrote. So, taking a one-off character and putting them in a main cast role in a new show would be providing creator credits for the entire show.

Chief Miles O’Brien is a different story altogether. Colm Meany was originally contracted as a background regular as the transporter chief but without a name. These kind of roles are reliable and keep actors eating while allowing them to audition for other roles.

According to what I heard at a Creation Con panel back in the early 90s, the showrunners eventually decided to give the character a name. Colm Meany wasn’t advised, nor was his agent — so Meany actually was raising a complaint that he was being replaced without notice, only to find out he’d been upgraded to a named part.