Tefillos of Erev Yom Kippur Selichos are shorter than usual. We don’t say Tachanun or Avinu Malkeinu because Erev Yom Kippur is a festive day. Mizmor l’sodah said in remembrance of theKorbanTodah isn’t said on Erev Yom Kippur because theKorbanTodah wasn’t sacrificed on this day. (TheKorbanTodah had to be eaten during the day it was offered up until midnight. On Erev Yom Kippur we have much less time to eat only until sunset so theKorbanTodah wasn’t brought.)