Everything is made out of plastic these days, isn't it?
From My Own Experience as a Sea Kayak and Canoe Guide in British Columbia
In the water, the material that a boat is made of doesn't really affect its handling and performance. A canoe (or kayak) can be made out of metal, wood, fibreglass, Kevlar, or plastic and have very similar buoyancy and "feel" the same as the same hull design made in another material.
Kevlar is the lightest out of the water so that's the easiest boat to lift off your car and carry (sometimes by yourself) down to the water. Plus Kevlar is a cool word to say and has good impact resistance. However, Kevlar has poor abrasion resistance. The difference between impact and abrasion is like the difference between paddling your boat as fast as you can into a rock and dragging your boat up a beach with sharp rocks. Both of these scenarios can happen intentionally or by accident.
Anyways, back to your question. You say, "aluminium canoes are more durable". This could be true, but plastic doesn't dent as easily after an impact and can be re-formed with a hairdryer (!!!). Yes, plastic boats can warp if left too long in the hot sun. Some other cons against an aluminium boat could be:
Some aluminium boats are riveted. These rivets can loosen over time making the boat leaky, depending on its design. Other materials can be formed into one piece (a whole boat). Fiberglass is actually very easy to repair.
I'm sorry to say this but aluminium boats are noisier. Bumping the paddle against the gunwale makes a lot more noise on metal than plastic or fibreglass, etc. This can be annoying to the wildlife in the area.
The Following Is More Speculation than Personal Experience
Manufacturing with lightweight metals may be more costly nowadays than working with plastic. If a product is made beautifully in wood or metal, someone out there is making it cheaper (and uglier) in plastic. It's sad, I hear ya.
Lastly, the look of aluminium is not as nice as the multitude of colors that a fibreglass/Kevlar (or even plastic) boat could be. Even wood looks better than metal. I realise that this is not a practical reason, but I do believe it is a reason we don't see more boats made out of metal these days. I also realise that some may like the look of metal over solid color. However, I feel that those folks are in the minority.
Overall, it doesn't really matter what your boat is made out of as long as it floats and you have fun using it!