There are a lot of factors that make this card worth so much compared to any other Charizard card. Specifically, the lack of a shadow means it isn't just a first edition, it's the first print run of the first edition. While most base set cards have a shadow on the right side of the art frame, the very first cards Wizards of the Coast printed lacked it, giving them an extra level of value.

Second, though shadowless Base Set Charizards are fairly common and number in the thousands, a PSA 10-graded one (meaning it is in mint condition) is incredibly rare. Only 56 have ever been identified by PSA.