Well, several reasons. It began with the Israelites wandering the desert for 40 years after hesitated on entering the promised land. Following this, both Holy Temples in Jerusalem were destroyed. The first by the Babylonians and the second by the Romans.
More tragedies continue. The Bar Kochba Revolt was lost, the Temple Mount was plowed, the English banished the Jews, and Spain banished them as well.
For these reasons, Tisha B'Av is an incredibly sad and solemn holiday. It is also a fasting day.