Why don't we say Avinu Malcanu on Tisha B' Av?
The Artscroll Siddur says "on fast days that tachanun is omitted...", is there any other reason?
The main reason that I recall is because of the pasuk in Eichah (1:15) that refers to the day as "moed"
קָרָא עָלַי מוֹעֵד לִשְׁבֹּר בַּחוּרָי
I don't remember why that forces a connection.
Edited to change my answer. Here is a link that explains why we don't say Avinu Malkeinu on Tisha B'Av.