Because Judge Greer has forbidden them to.
He & Michael Schiavo both belie their intent, motives, and prejudiced state of mind by not ordering the hospice to see FIRST if she could eat soft foods and drink by non-artificial means before carrying out the directive to pull the tube. Often, tube feeding is for the convenience of the hospice or home --- too labor intensive to feed by hand, especially if the disability precludes use of a cup or eating utensils.
Why didn't it ever occur to anyone to try! Now that the tube is pulled, how can it be lawful to not try to give her sustenance by non-artificial means. If a ventilator is pulled a person breathes or they don't. Air is present to be inhaled. If a feed tube is pulled, a patient should be presented water and food via a means to ingest it. In Terri's case, a sponge. Why? Because she did it before, and it hasn't been tried since. It may not work now, but how is it possible it has not been attempted as a last resort.