The hydro power is located in the far north(west), meaning that there is a bottle-neck in the hundreds to thousands of km of transport lines needed to bring it to the big cities in the south.

The capacity of these lines is simply not enough to supply the cities in the south with only hydro power base load, which is why Sweden had the four major nuclear power stations built in the south, in the vicinity of major urban areas.

It was a really well thought out system, that has been degrading over the last years for political reasons.