Let's assume everyone's buying tickets at a reasonable price, €20 (€40 return) one way from Dublin, €79 (€158 return) one way from Waterford.
For 1 person, bus to the airport from waterford, €30 Dublin coach (free for kids U12 with adults) That's €70 return from Dublin, or €158 return from Waterford for one person.
Fair enough, waterfords only a little extra and it's a little easier to fly from there.
Now, a family of 4, that's €280 return from Dublin, or €632 from waterford at the lowest fares. This is where it becomes very uncompetitive. This is why regional airports find it hard to compete, same reason as people from Shannon and Corks catchment use Dublin for places like Birmingham and Edinburgh.
That and 6x weekly/3x weekly verses several times daily from other airports.