On net income of 100K, I would expect to be able to receive tax credits up to 75K donations. Why does turbo tax advise carry over to next year at half of that ?

The charitable tax credit is a non-refundable tax credit that you can use to reduce the tax that you paid or may have to pay. It can’t be used to generate or increase a refund. So, if you don’t have any taxes payable, you will not get the credit. That is the most likely reason why TurboTax is recommending that you don’t claim the full amount of your donations. You can carry forward donation amounts for up to 5 years, or if you have a spouse or common law partner, they can claim some if they need it.