I'll let Dan respond as to why he would or wouldn't do this. As a former operating dept guy I'll throw my couple of cents in.
When you make a "special event" an everyday occurance, it's no longer "special". It doesn't matter if its the Eureka, Goose 5 or the Rotary, if it starts to happen all the time, people loose interest. The D&S has managed to come up with something a bit different each year for Railfest to continue interest in the event.
In reality, the operation of the Eureka istelf probably doesn't make much money - if any. It's the attraction and other stuff to see and do that draws people in. If the Eureka were to run everyday, a paid crew would be required ($$$$) maintainance would be required (more $$$$). I know steam mechanics well enough to know, there are VERY few people I'd trust to wrench on it. With a capacity of only 44 people a trip (one coach), there would'nt be enough revenue to pay for itself. Getting a continual supply of wood and storing and spotting that online is another concern.
When we ran the Eureka on the C&TS we made a little money. We did so because we ran it for over a week, did a variety of trips, and could handle up to about 50 passengers a day. If I had done it again, I would have had to come up with something "new" to get people to come back a second time.