Why is PACE (Property Assessed Clean Energy) capital not a loan?

Because PACE is a special assessment. (Investopedia) "A special assessment tax is a surtax levied on property owners to pay for specific local infrastructure projects such as the construction or maintenance of roads or sewer lines. The tax is charged only to the owners of property in the neighborhood that will benefit from the project. That neighborhood is called the special assessment district"

PACE is a surtax.
 It is not a loan. It is not borrowing. It is not your money.
 It is a neighborhood of 1 property: YOURS!
It pays for specific infrastructure benefitting the larger community environmentally. The infrastructure is chosen by you.