They didn't continue building the Confederation class because they couldn't afford to in the ending years of the war. The escort carriers were built so that Confed could have a fighter presence in multiple systems without needing full carriers in those systems. By the time they had the ability to continue manufacturing new ships after the war, there were already better designs than the Confederation in use, like the Vesuvius.
Edit: It also could have been a combination of this and Quarto's reason above.
Edit: It also could have been a combination of this and Quarto's reason above.