Maritius has a smaller population, as does the seychelles .

Much of the modern economy (last 50yrs) of Maritius has been built on tourism since there are few economic resources and it is isolated .

The seychelles were much more agrarian that the small population of Mauritius, and they produced plantation crops well into the 1970s, similar to a lot of developing countries . Vanilla bean productions, copra .

Now why are they richer?

Obviously having more of a small coastline and location are parts of it.

The Seychelles are closer to the Red Sea and Indian Ocean trade route, and lack the rough terrain/hills, and isolation that Madigascar has struggled with (look at the erosion and road problems). Also, on this note, with a non infinite amount of travelers, it was easier to pull people out of poverty as the tourism industry developed in the 1970s . This couldn't happen say, in Madigascar . The whole small population benefits also were more potent because the resources of the island (Seychelles) were spread across the small population, so take fishing for example. There are only so many fish in the sea, and the small population made the carrying capacity and realative wealth per person greater on a per capita level.

Both nations either retained or had very fair legal amd governmental policies, something Madigascar lacks (lots of vigilantism, similar to PNG this way) .While forced labor and the plantation economy did exist on both nations, they were safe and accessable to get European tourists and some offshore banking there starting in the 1970s .By comparison, a year or two ago there was an acid attack on a girl in Madigascar on vacation (this happens from time to time in Zanzibar and Madigascar) .

Maybe state institutions took better hold because of the small population during their respective colonial periods? The interior of Madigascar had little investment or buy-in towards the institutions of french governance.

British institutions in the case of Mauritius may have helped with pluralism. Speaking a creole french and english in the Seychelles and Mauritus certainly helped with tourism over the more difficult native tongue in Madigascar.