Looking at 5E spells, demons seem easiest to summon but hardest to control, and devils are more difficult to summon but can be cooperative, but yugoloths don't have any spells dedicated to summoning them.
Extrapolating from this, it seems that either A) mortal spellcasters don't have much interest in summoning extraplanar mercenaries, or, more likely, B) yugoloth mercenary services are prohibitively expensive for most. Someone who wants yugoloths working for them has to plane shift over to their domain themself to hash out a deal in person.
"You can hire me for 500. You can trust me for 10,000 more." - Mezzoloth quote from FR Wiki