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.
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.