Why is Ponder banned?

Were you asking about Modern/Extended?

Ponder and Preordain
A large number of blue-red combination decks kept the field less diverse. One thing that made them so efficient was the cards that would find their combinations. Ponder and Preordain were the most widely used of those cards. Banning these should make those combination decks somewhat less efficient without removing the possibility of playing them.

Extended

Jace, the Mind Sculptor; Mental Misstep; Ponder; Preordain; and Stoneforge Mystic are banned.

For the past year, no cards have been banned in Extended. The outcome was that the best Standard decks over the preceding years, such as Caw-Blade and Faeries, were the best decks in Extended. Many people who would otherwise enjoy Extended did not enjoy that environment. To avoid repeating this, the DCI is banning the cards that have been most played in the most successful Standard and Extended decks over the past three years. For reasons similar to why these cards were banned in other formats, the DCI is banning them in Extended.

Because it provides very good top devk manipulation which can lead to very explosive draws. Depending on what format your referring to, it might have been baned to help slow down the meta so players can have more fun.