It’s simply this.
If you think that growing healthy plants in a freshwater environment is challenging, it’s more challenging growing healthy live corals.
Lighting is important in a planted fresh water tank, but infinitely more crucial in marine.
If maintaining water chemistry is complicated on the freshwater side, it’s more complex with a marine environment.
If fish choices are difficult, the same applies.
There are just so many more variables to stay on top if in the saltwater world, and many “tight” requirements.
And then of course, there are the dollars involved. More equipments is required, and that dedicated equipment is more expensive.
Everything is more expensive, the corals, the fish, the chemistry, just everything.
But if you are looking for a challenge, and have the requisite time and desire to devote to it, then it can be very rewarding too.