Actually, it wasn't. The stressors involved with bringing elephants to a public event endanger the safety of both the pachyderms themselves and the humans around them. First and foremost, urban stimuli -- the lights, noises, strangers and pavement -- all place strain on the elephants, who can become aggressive and potentially dangerous when agitated. Additionally, the elephants (Suse and Esa) were transported to Dallas from Oklahoma, a long distance that necessarily must have kept the elephants confined for an extended period of time.
And, in addition to having to endure the stress of an unfamiliar environment, Suse and Esa live in captivity at an undisclosed farm in Oklahoma. That means it's likely the pair has been trained to comply with humans' orders with the help of a bullhook, a cruel tool commonly employed to teach elephants total subservience. (Sure enough, images from the event show trainers standing alongside the elephants with bullhooks in hand.) Individual cities have moved to ban bullhooks, which habituate elephants to horrific pain, across the country; they're still effectively legal in Texas and Oklahoma, though.