It's probably because of its relative ease to proc, and you're not gaining much by waiting for the cooldown.
With a rune like Manaflow Band, waiting an extra second or two to use your ability means you gaining a permanent +25 mana, and the faster you stack that the better. It also triggers only off of abilities, so if you use your one ability that can affect enemies right before it's up, you have to wait an extra several seconds before you can try to proc it again. Similarly, you need to know the cooldowns of Dark Harvest, Electrocute and Comet so you know how available your burst damage is. In the case of comet, its cooldown changes when you land other abilities, so it would be impossible to count down on your own.
However with taste of blood, you can trigger it off of auto attacks, and its cooldown isn't unreasonable. If you use your combo right before the CD is up, you can always just throw an auto attack at the enemy champion to get your little heal. It's never a life or death situation, while with other runes who deal damage, it almost always can be. Other runes can become essential to a champion's abilities and playstyle, like Lethal Tempo, Phase Rush, Electrocute, Aftershock, etc., but knowing whether you're going to heal 20-80 health with this trade or not is not really essential information. Cluttering up the hotbar can be a real problem.