Supply and demand are both important; however, demand trumps supply in the long run. There are other very important considerations, of course, when determining the extent of "demand".
If we are talking about a particular set or a player, how many collectors collect the set or the player? If there's substantial number of collectors for that set or player, then we turn to supply. Within supply comes the matter of examining the number of examples for each grade. One of the key factors with the demand will be how beautiful was the set's design and for a player, how does his card look---stunning, amazing, breathtaking OR humdrum, raunchy, or flat-out ugly.
The brother that mentioned the addition of having an enthralling story to go with the set, and therefore a player from the set, is thinking wisely and offers striking examples that back his claim of its importance.
Distilling all of this, when you have a set or player that collectors love, and the supply of high-end graded examples are few, or the supply of collector-grade examples seem to take months and months to locate, the price point will keep rising.
Many regional / food cards and team-issue items are sleepers, for their supply is disturbingly low; however, the far majority of collectors seem to gravitate to the tried and true gum cards. Their design is usually appealing; their availability is easy to locate; and they don't have to think too hard or deeply whether or not they're worth collecting.
Whereas, the attractive regional / food items, with their built-in scarcity, rarity, and a paucity of a pop report, often seem over-looked and under-appreciated, for the collector bandwagon does not enjoy having to think for themselves, work hard at researching, and works even less still at tracking down the elusive.
Then again, the little secret is that the few collectors who long ago got their gum card fix, and then decided to launch out and go after better game or white whales, entering the narrow path full of obstacles to get to the gorgeous, tough-to-get regional / food cards would prefer, and I mean adamantly, that the gum card collectors who constitute the far majority of today's collector base, would just remain happy and content with their gummies----FOR THEY DON'T WANT ANY MORE COMPETITION FOR THOSE ITEMS; LIFE'S TOUGH ENOUGH COLLECTING THE BLUE DIAMOND REGIONALS!!!
Just some "food for thought"......
Oh yes, since other companies are forbidden to release regional / food cards today, and have been so for decades, that is why the current legal sport card makers have produced their manufactured rarities, or "chase cards", to replicate in their own way the original, all natural chase cards---those same aforementioned difficult to get regional / food items----tough to get back in the day; often tougher to get today.
Supply or demand. I say both, but mention demand first.
Stay safe. Drive carefully, as many seem to have had their driving skills affected by the Coronavirus and its accompanying fear factor.
--- Brian Powell