Question

Why wasn't the convention changed after it was discovered that electric current is actually the flow of electrons

Answer 1

Initially when discovery of electricity was in progress it was widely propagated that flow of current is same as flow of proton. But after the discovery of electrons by Thompson it was rectified that current flow is actually the flow of electrons.

The conventional method current flow was taken arbitrarily and wasn't discarded because many theories have already been proved by considering the conventional current flow and were globally accepted like like Fleming's right and left hand rule.

So, putting it in a nut shell , the conventional method was put forward without any valid reason. But direction of cureent flow never matters in determining any quantity so never confuse your self with direction and concentrate on the polarized terminals.