The Guild of Bards was poor. Very poor.
The Guild of Mercenaries, Guild of Assassins, Guild of Rogues were all wealthy, or at least could make ends meet. The Association of Mages was even full of magnates.
Most of those guilds could accept contracts and they were very strong. Therefore, their members could find jobs anytime, providing taxes and revenues for their guild.
As for the Bards… Well, they weren't capable fighters, and they weren't as good at support as Priests. Unless it was absolutely necessary, no mercenaries would ever invite them on an adventure.
Besides, while their teammates were fighting life-and-death battles, the Bards would be singing and playing musical instruments behind them… Their partners couldn't be happy even if they won a battle.
More importantly, the enhancement of the Bards' performance was indiscriminate, which meant that the enemy would be buffed too.
Therefore, they were really not competent as teammates.