When the more than 15,400 tickets sold out in about 90 minutes to Pullen Park's Holiday Express on Tuesday, there was a common question among disappointed parents, who didn't manage to snag the coveted tickets: Why can't they just add more days?

Here's what some were saying on Go Ask Mom's Facebook page:

"I don't understand why Raleigh Parks & Rec doesn't make the event run daily for the entire month of December so more people can attend. They must know the event only gains in popularity each year. Don't they want as many people as possible to be able to attend? Instead it ends up being an annual frustrating disappointment for 90% of families who try to attend. Extending the event also increases their revenue," wrote one woman.

Wrote another: "If it's this popular, I'm surprised they don't add another weekend to it."

"You'd think!?!? seems like an easy way for them to make money," somebody replied.

But, adding more days isn't so easy, Kellee Beach, marketing communications administrator for Raleigh parks, tells me. There are lots of logistical challenges.

This year, the city did extend the event. In 2016, after the original eight days sold out within hours of going on sale, the city added a ninth day to the event, which also sold out. This year, the city scheduled Holiday Express to run 10 days (on its 10th anniversary). It's set for 4 p.m. to 9 p.m., Dec. 7 to Dec. 10 and Dec. 12 to Dec. 17.

Here are the four biggest obstacles to adding more days:

Securing staff and volunteers already is a challenge

Each night, it takes 65 staff members to run the Holiday Express. That includes 45 city employees, along with 20 volunteers, often groups, families and individuals, from the community. Some of the city employees work at Pullen Park and, for instance, operate the train and carousel. But many of them work elsewhere at other city parks and are pulled away from their regular jobs to help out with the event.

Santa is booked

The meet and greet with Santa is a highlight of the event, which also includes lots of holiday lights, music, crafts and rides on Pullen's kiddie train and carousel. Many vendors are booked almost a year in advance. In other words, Santa and his helpers are very busy at that time of year.

General public couldn't play at Pullen Park

The Holiday Express runs 4 p.m. to 9 p.m., nightly, but Pullen Park is actually closed each day of the Holiday Express to control entry into the event and to protect the amenities, equipment and activity stations that are part of it. If, for instance, the Holiday Express ran through the month of December, as some have suggested, the park would be closed to the general public throughout that period.

Swimmers, clubs and other groups also impacted

Pullen Park isn't the only facility that shuts down to the general public during the Holiday Express. Pullen Aquatics Center and Pullen Community Center also close early each evening of the Holiday Express to accommodate the event. Beach said the city also doesn't program swim lessons or other classes during those hours - losing out on revenue and disappointing regulars to these facilities. The pool also is closed to lap swimmers and swim teams who practice there. What's more, the city also must move community groups and clubs, who meet at Pullen Community Center, to other facilities across the city during the event.

Of course, Holiday Express is NOT the only Christmas event in Raleigh or the Triangle. I listed many of them in an earlier post this week. If you don't have Holiday Express tickets, there are plenty of fun options available.