What? That doesn't make sense at all.
The one that came later has the burden of compatibility.
The issue is then why doesn't the PS2 memory cards have the same kind of compatibility as PS1 cards?
The answer is because they're structured differently; PS2 cards are internally (inside the case) different cards; even if externally they're structured similarly.
In that case, why couldn't they do something like have the PS2 capture save I/O data, convert it to a PS2 card friendly format, and similarly convert PS2 card data into PS1 friendly format?