Most of the exoplanetary systems discovered so far have large Jovian planets. Why is this expected?

They're the easiest to find. We use a couple of methods in determining whether a star has planets - one is the slight drop in light as a planet transits across it's visible disk, the other is to see a planet 'wobble' slightly, due to the gravity of a planet tugging on the star.

As such, the larger the planet, the more obvious the drop in light would be in the first method, and the more the star will 'wobble' due to it's larger gravity. A plus is having this large planet close to the star - a so-called "Hot Jupiter" - because the star wobbles faster, and the periodic drop in light is recognized faster.

It takes a big planet to be detected at this distance. You can see the house down the street, but you can't see the spider climbing up the wall, even with a good telescope.