Miami is a football city. They love, love, love the Dolphins.
Why? Tradition. Super Bowls. The Perfect Season. People, when they are young, learn to be fans of certain teams. And the Dolphins were winning in the 70's and 80's, people grew up and love them. My family has been a Dolphin season ticket holder for 15 years.
Then everything was expansion. With expansion comes no tradition. You need to breed the young fans by winning so as they grow older, they remember those days of winning and hope for them again. The Heat never won anything so no young fans grew up to be a fan of them. People are obviously coming around now, but it is the young generation, the impressionable kids, that will grow up and be Heat die hards now. The Marlins won something, but then had a firesale and killed any momentum they had going.
But besides that, the stadium is absolutely garbage for baseball. I mean it is a football stadium, plain and simple. When you go to the ballpark, you want to have a baseball experience, and that stadium doesnt offer it.
The weather is a big issue. especially when you consider where the stadium is located. It's located in the middle of nowhere. This is fine for football, because its once a week and not in the rainy months. But when you need to travel 40 mins to get to the game from all directions, and there is a huge chance of rain, and the stadium sucks for baseball anyway, and the team sells you out and firesales off the team everytime, and you dont have young diehard fans that have grown up with the Marlins, it is tough to draw fans.
Miami is a place with A LOT of things to do. And in order to make the ballpark a good experience, it needs to be surrounded with bars, restaurants, nightlife, something so that people can make a day out of it. Right now, you go to the yard and it is surrounded by a huge parking lot and houses. In Boston, you have restaurants, bars, whatever right around the stadium. You have a mode of public transportation. Its located in the city.
Overall, if the Marlins build a stadium closer to downtown, so it has some nightlife or restaurants around it, and they put a retractable roof on the stadium so if it rains, people dont waste their time, and they commit to winning and stop with the firesales, people will begin to like to this, especially the younger generation and be fans of them.