Agar can be solidified at about 43 degrees Celsius, and the concentration of gelatin is mostly liquid at room temperature, which is not easy to solidify.
Some bacteria can liquefy gelatin (gelatin liquefaction test, because the concentration of gelatin used as culture medium is liquid, it must be frozen in the refrigerator for observation)
Agar is more nutritious.
Agar is cheaper.
Back white-reagent library-agar.