Scald is great because it's able to be used for more than just the burn chance. It's distribution is great, with pretty much every Water-type being able to learn it. It's also only a 10BP drop compared to Surf, which it more than makes up for with the burn chance. Only 2 abilities are immune to Water as well, with no types granting immunity.
Meanwhile, Spicy Sand has absolute garbage distribution, with most Ground Pokemon being physical attackers that can't make use of it, and an entire type, item, and ability grant immunity to Ground-type moves.
Some Fire Pokemon get the move, but even most of them are physical attackers. Since Gamefreak hates giving certain types moves, it's appreciated coverage for them, but even most of the Fire-types that get it are physical too. The Pokemon that can make proper use of it over other moves are shitmons like Pallosand or Turtonator.
