The tabla provides the rhythmic base for the entire performance. Since the main performer will constantly refer to the tabla, it is essential that there be a conventionally established pattern that may be universally understood. This pattern is called theka. Too much variation from the established theka may lead to a breakdown in communication and thus compromise the entire performance. In the old days, vocalists and instrumentalists would not allow their tabla players to play anything except theka.
Today there is much greater freedom, still the basic responsibility remains.
The role of the mridangam is much different. The mridangam does not have to provide the rhythmic base for the performance. Such a base is provided by a conventionally established pattern of claps and waves. Half of the audience in a south Indian performance may be clapping along with the performers. With so many people providing the base there is a greater freedom given to the mridangam player.