Answer:
because they are Delicious
The real reason is that latkes were originally made with cheese. This was to commemorate the heroism of Yehudis (Judith) during a siege at the time of the Maccabees.
She tricked the Greek general into eating lots of cheese and drinking wine, until he was unconcious.
She then cut off his head, placed it in a sack, and returned to the Jewish city, where the soldiers displayed the head on top of the ramparts. This so demoralized the Greeks that they fled.
Eventually, the custom of cheese latkes evolved to include potatoes.
For more details, see chabad.org, where I got this answer from.