The reason is obvious isn't it? Without the tax credits and subsidies, the public option would be the only affordable health insurance option for those persons who are mandated to purchase insurance because they do not have other insurance access (employment, veterans, etc.) the last thing Republicans want is for folks to migrate to public insurance options. Indeed, regarding Medicaid expansion, many red states have insisted on making Medicaid a private option affair.
It's worth considering whether Democrats should start arguing for adding a public option as the "fix" should the SCOTUS rule for the challengers in King. One of the reasons this works (and would have worked to forestall the King challenge) is that, in contrast to private insurance, which requires the government to transfer funds to it to cover the costs of private insurance purchased on the exchange (this is the "affordable" part of the ACA), a public option would not need a government transfer of money, since it is the government itself which would finance a public option. No tax credits or subsidies for the public option? No problem, it's just accounting entries anyway - from one government ledger to another.
It's time to consider the public option again.