"The island can be accessed by public transit, by car, by bicycle or by foot. The Concordia Bridge links St. Helen's Island to Montreal's Cité du Havre neighbourhood on the Island of Montreal as well as Notre Dame Island (which itself is connected to Saint-Lambert on the south shore by bicycle paths). The island is also accessible via the Jacques Cartier Bridge from both the Island of Montreal and Longueuil on the south shore. The Yellow Line of the Montreal Metro has a stop on St. Helen's Island: Jean-Drapeau station."
When I lived in Montreal, St. Helen's island was and is very accessible. Maybe Toronto Island(s) and St. Helen's Island are not comparable? No, St. Helen's probably doesn't have an airport.
Update: Ok, I refreshed my memory. St. Helen's sit between Montreal and the South Shore whereas Toronto Island(s) sit between Toronto and water.
The geography of the two areas are quite different. I don't know if St Helen's is affected by flooding, but it's less exposed like the Toronto Islands. Montreal chose to build various attractions on St Helens island, which makes the bridge and subway station worthwhile. These connections allow people to move between Montreal and Longueuil, which is a logical rationale for building them in the first place. These conditions don't exist on the Toronto Islands.
We certainly could build up the islands with housing or attractions that can operate year round like another museum, the Science Center, Ontario Place, aquarium, etc. but that's very unlikely to happen. A tunnel / bridge only makes sense if you have enough people living or visiting there.
For reference, that tunnel connecting the foot of Bathurst to Billy Bishop costed almost $100 million. So a pedestrian tunnel to the rest of the islands would be immensely expensive.