Hi,

The likely reason for this is that the use of antidepressants are known to tip people with bipolar towards mania. So, despite the distressing symptoms that accompany depression most doctors prefer to use medication that stabilises mood. Having said that there remains controversy over the use of antidepressants. Some have argued that giving an antidepressant has no effect at all, yet others say their use may be warranted if depression is protracted and unresponsive to mood stabilizers alone. Depending on the particular circumstances it may be felt that the benefits of giving antidepressants outweigh the costs.

You might find this short video called relieving bipolar depression interesting. This article on Bipolar Disorder Treatment discusses the options in a little more depth.