months - Why the difference between Tishrei and Nissan for tachanun?

As mentioned elsewhere on this site, we don't say tachanun during the entire month of Nissan, because a majority of the month is spent "in kedusha," skipping tachanun -- therefore we omit tachanun for the entire month.

Don't we do the same during Tishrei? Based on the above reasoning, why should we say tachanun at all during the month of Tishrei? (assuming that we don't say tachanun after Sukkos)