why are pterosaurs and aquatic marine animals in the past not considered dinosaurs?
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Because they are not in the animal group that we call Dinosaurs. We classify biological organisms in groups where every gin that is decent from X is in group X. What we called Dinosaurs are descendent of a specific form of life and those decedents do not include the large marine animals of that time.
A distinguishing feature of dinosaurs is the placement of the legs. Dinosaurs have legs beneath the body as mammals do. Compare to crocodiles that have their legs sticking out on the sides of the body. Look at [this drawing](https://en.wikipedia.org/wiki/Terrestrial_locomotion#/media/File:Sprawling_and_erect_hip_joints_-_horizontal.svg) where crocodiles are the sprawling types.
It is not different then Crocodiles are not dinosaurs but both are reptiles. Crocodiles are in fact closer related to Dinosaurs compared to the Plesiosaur and Ichthyosaur that lived at the sea at that time. You can look at how are related in [this image](https://qph.fs.quoracdn.net/main-qimg-13271ae705ddc6d3dd5f9a46d178a575). The flying Pterosaur is close to dinosaurs the dinosaurs are to Crocodiles.
So if you would like Plesiosaur and Ichthyosaur to be included in what we call Dinosaur you have to include Crocodiles too. But that group’s od already have a name, you could pick for example pick Diapsids or even Reptiles
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