This is sort of a theory, but I was reading The Time Paradox and I came to this passage:
Eight Years Ago/Two Months After Artemis Sr's Disappearance
'... [Butler] glanced across at Artemis and was surprised to see him dressed in one of his two suits, the dark blue one he had worn recently to the opera in Covent Garden. Artemis had always been a neat dresser, but a suit and tie were unusual even for him.
"Are we going somewhere formal, Artemis?"
"Nowhere formal," said Artemis, with a coldness in his tone that the bodyguard had not heard before but would come to know well. "Just business. I am in charge of the family affairs now, and so I should dress accordingly."
"Ah... I detect a distant echo of your father."
In short, Artemis started wearing suits because he was in charge of the family, and perhaps to remind him of his father. Just a theory, but I think that's why Artemis in the film didn't really wear suits, because of the terrible change of plot.
Then again, I'm almost certain nobody on the team read the first book, and definitely not up to the fifth book, and the more likely reason is that they just didn't bother. But it's still quite interesting!
Artemis Fowl: The Time Paradox, p. 71-72, paperback