Don’t confuse Hesped and Tachanun. In Nissan, we do not fast or make hespedim, and also don’t say Tachanun.
In Tishrei – from after Succos, there is only a minhag not to say Tachanun because most of the month already past without tachanun. Hespeddim and Fasts are still practiced.
I am just saying what I learned from the Mechaber, Remah, and Mishne Berrura. I am not claiming to understand it – is it in fact Rov?