In school life we were taught that $<$ and $>$ are strict inequalities while $\ge$ and $\le$ aren't. We were also taught that $\subset$ was strict containment but. $\subseteq$ wasn't.
My question: Later on, (from my M.Sc. onwards) I noticed that $\subset$ is used for general containment and $\subsetneq$ for strict. The symbol $\subseteq$ wasn't used any longer! We could have simply carried on with the old notations which were analogous to the symbols for inequalities. Why didn't the earlier notations stick on? There has to be a history behind this, I feel. (I could be wrong)
Notations are notations I agree and I am used to the current ones. But I can't reconcile the fact that the earlier notations for subsets (which were more straightforward) were scrapped while $\le$ and $\ge$ continue to be used with the same meaning. So I ask.