Dry

Why did some freshwater fishes decide to move on to the shore but marine ones didn't ?

If you think of it, the answer is simple: Ocean is such a big and nice place that its inhabitants never needed to consider any drastic move. If a part of the sea becomes not suitable for their subsistence, the fishes could always swim to somewhere else.

But, the environment of freshwater is different. Many lakes dry up intermittently - leaving just an area of swamp; rivers often run a slightly different path every few years and the level depends very much on the rain fall (or snow fall). Vertebrates swimming in this kind of environment are forced to get up on (alas, develop) their own feet.

PS. Of course many fishes could not adapt to the change and died of dessication. It was fortunate that our pantheistic god is not some modern hospital administrators - otherwise these terrestrial beginners would be confined to the water forever because their apparatus (alas, fins) would always be considered not suitable for walking.