Why is crossing the Delaware quarter rare?

I'm curious, why is the crossing the Delaware quarter considered to be such a rare coin? Is it due to a limited mintage during its production? Or is it because it was never widely circulated, making it harder to come across in today's market? I'm also wondering if there are any particular features or historical significance attached to this quarter that contribute to its rarity and value. Can you shed some light on the reasons behind its exclusivity and what makes it so sought-after by collectors?