Mostly practical reasons probably. In the movies, Twi‘lek were background characters. Visible for a few scenes each with not much to do.
Now you have characters who are on screen and therefore on set for a lot longer and have to do more action scenes. Having long and wobbly lekku would be pretty exhausting for the actors and in the worst case they‘d get in the way so much that they would have to reshoot scenes until they behave as expected.
Costuming department couldn’t be arsed. Same reason Ahsoka still has the montrals of a 14-year-old even though adult Togruta are supposed to have them, like, down to their knees or something.
But why do all Twi’lek in the new Disney shows have such short and rigid lekku, compared to the movies?
Mostly practical reasons probably. In the movies, Twi‘lek were background characters. Visible for a few scenes each with not much to do.
Now you have characters who are on screen and therefore on set for a lot longer and have to do more action scenes. Having long and wobbly lekku would be pretty exhausting for the actors and in the worst case they‘d get in the way so much that they would have to reshoot scenes until they behave as expected.
Costuming department couldn’t be arsed. Same reason Ahsoka still has the montrals of a 14-year-old even though adult Togruta are supposed to have them, like, down to their knees or something.