Because at the moment vast numbers of people are paying twice for services.
If an individual chooses to replace the (inadequate) service provided by the state via tax payments out of their own pocket then these costs should be tax deductable.
For example if you pay for private healthcare or education then these costs should be recoverable from the state as in effect you have paid for the service twice. Once to the state via tax (which presumably has not provided the level of service paid for) and then once again through the private payment.
This is papably unfair.