From where does this "tendency" originate? Here are a few profiles:
Risk aversionaries. Many forget how highly-touted Grissom was a year ago; the Braves GM called him a future All-Star. As long as injury or illness kept Gris from playing, he was an unopened wax pack. What was inside: star cards or commons? Collectors know most vintage packs, once opened, lose all value...
Business minds who favor keeping a prospect down long enough for the team to secure an an extra year of control over the player... who, chances are, won't even be with the same organization six years from now. These types of fans must be relatively young and healthy -- unlike me, who doesn't even know if I'll be around an extra six months from now (thus, my urgency to see the best possible Sox team all season).
Worry warriors, who don't want to rush a kid into the Big Leagues -- "don't ruin him for life!" But... pro baseball executives (not fans) deem players ready for The Show. Prospects earn promotions, and then have to make constant adjustments to prevent demotions. Dalbec bounced up and down between Boston and Triple A for six years -- who ruined him? Answer: Major League pitchers.