Because the benefits of SPS inure directly to stakeholders, and only indirectly to those whose work creating, curating, and witnessing create all value Steem possesses. Do you want VP to be based on rewards? That would be silly! VP is based on stake, and polls to effect application of SPS will apply VP. Improvements will produce price impacts on Steem, which is directly benefiting stake, not rewards.

Why dodge the reality that the authority to vote SPS is based on stake, and therefore the responsibility to fund SPS should be based on stake?

If rewards are taxed, the incentive to create benefits are reduced. Stake which does not create benefits then gets a free ride, which increases the relative power of stake which neither creates, curates, nor witnesses. Do you really want to provide incentive for stake to do that? Because that's how you do that: tax rewards to fund SPS.

The benefits of SPS inure directly to stake. VP which will determine SPS application is based directly on stake. Since both the authority to effect SPS and the benefits of SPS are determined by stake, so the responsibility for funding SPS should be based directly on stake.

Don't decrease incentive to create incentive for investors. Create clarity and control for investors to produce SPS funding that produces the gains they seek.