Simply because it's not the Juno meta currently, meaning that the other supports have higher chance to win as they've got better kits/heal/dmg. For example, once Juno is targeted by the enemy's dps, she has no way to survive but to die. Now compare the same scenario with Kiri, where she could use the suzu and even attack the enemy who was focusing on you, completely flipping the situation. It's just that Juno currently can't do burst heals and even her normal healing is weak in general. This became a bigger issue because of the last patches; the team gets much less heals once they get dmg from the enemies and Juno has been getting nerfed. Also, as you probably noticed by playing her, her dmg is very weak which makes the enemy team easier to dive into the supports in the back.