During the playoffs and championship series I went to in my area
attendance was way below the regular season attendance average. There
are several reasons for the decline during the most important part of
the season...

-football season has started for high school/college/pro.

-school has started and families can't make it out like they did during
the summer.

-advertising of playoff games is slim to none so if it isn't in the
pocket schedule or yearbook people don't know about it.

-some season ticket holders only buy regular season tickets and don't
want to pay more for playoffs.

-regular season games are used as a gathering place and only the
die-hard fans go to the playoffs.

-some don't go because the star players during the regular season have
been called up to a higher level or leave because of a prior committment
(Chin-Feng Chen) and it's hard to rally around someone brought up from a
lower level to take his place.

-the city has no affiliation to the teams involved. Look at the
attendance to the Triple A Championship Games in Las Vegas on ESPN2.