I doubt everyone Coby saved at RP travelled around the world telling everyone to call him a hero, neither did the people involved in Rocks incident in Harp's case. The world at large is made to believe that Smoker's the saviour of the largest known country in the verse but literally no one called him a hero. Oda's clearly being inconsistent here.
It's not about them travelling around the World, telling people. Alabasta is one of the biggest and most renowned Countries in the World. I'm pretty tens of thousands of people visit the island regularly. And then you do have people from Alabasta who would travel around the World.
And regardless of that, what happened in Alabasta was an isolated incident. It does not garner the same recognition as the Rocky Port incident, which once again, is World renowned because of the people involved.
The World Government did it's best to keep the details of the events of Alabasta covered up, because it involved a Shichibukai, whom at the time, were considered allies of the World Government. They're obviously not going to glorify the entire thing all across the World, because at the end of the day, it would just make them look bad. Especially if they dubbed one of their own Marines a "hero" for protecting a Country from a Pirate that they sanctioned, and allowed to ruin the Country in the first place.
If every isolated incident like Alabasta was enough to dubbed a Marine a "Naval Hero" then there'd be hundreds, if not thousands of "Hero of the Marines" walking around.
The Rocky port incident is clearly a special case, because we are still hearing about it, and Oda himself is making it out to be a big deal. There's not been a peep about what transpired in Alabasta, anywhere in the World, since it happened.