Work order question

So posted a public work order on a character, and got a whisper from someone asking me to change it to a personal work order, my question is, why can’t they just fulfill it as a public work order.

Crafters have a limited number of public work orders they can fulfill and they may have run out. I believe the limit is 4 per day.

Four on Tuesday. Once used you incur a cooldown and it takes about a day before another one becomes available to you.