There weren’t a whole lot of choice games nominated in the first place, as shown in the infographic on the “Meet the Contestants” thread. Only 11.4% of nominees and 9.4% of contestants used Twine, the 12.5% of the Sweet Sixteen is in line with those figures.
The current breakdown by system is:
system # games
-------- -------
Inform 7 7
Inform 6 3
TADS 2 2
Twine 2
Inform 5 1
multiple 1