1. Because it is predicated on force.
2. Because charity isn't even really possible when one is forced to give, and I WANT to be charitable.
3. Because it deprives me of the ability to choose who receives my donations, and who handles the distribution.
4. Because it makes people dependent on the state, which is an instrument of violence.
5. Because it aggrandizes and empowers the violent and avaricious political class and the bureaucracies associated with it.
6. Because it creates government-run monopolies and deprives people of choice in who provides their needed services.

I could go on, but hopefully those reasons suffice.