Why? First, it works well for both desktop and mobile devices. Second, it is an open source, which is great. And this is probably the thing I should have started with. It is simple and by simple I mean it is very easy to start using it even if you don’t have any experience with maps. The API documentation is wonderful, the code is clean, readable and OOP-based and there are also tones of tutorials on the net.
With Leaflet I love that you are able to edit the CSS of all the map elements and customize it according to your needs. It is also very small and fast. Despite being about 34 kb, it has all the features most developers have ever needed for online maps. And it is easy to add features or customize existing ones to your needs.
There is so much that can be said but I think I should stop and go to the example.