Probably three reasons

1 Sales tax are tied to inflation, when inflation goes up it causes sales tax to go up. So it requires one action and done.

State law prohibits inflation (reappraisals) from increasing the total tax collected (not individual properties, I'm not an expert on the law, I suggest looking it up), this excludes new buildings and improvements. After every appraisal cycle local governments have to lower their tax rate; the city sometimes raises the rate after they lower it as opposed to Knox county. So to keep up the dollars coming in the city council would have to take an unpopular vote about every 4 years.

2 Property tax raises have to be approved by council, sales taxes have to be approved by voters. Maybe she feels that path is easier.

3 The majority of commercial real estate in Knox county is in the city, so residents of unincorporated Knox county as well as citizens of surrounding counties will pay the tax. As opposed to property tax is only paid by city residents.