So, I am starting to wonder why more people do not run 
on 
.
It amplifies her attack speed even when she heals someone else, gives her some AP, and it adds even more magic damage to her on hit effects. I get that it seems like a passive support item, but it is an all around boost for Kayle, including to her survivability.
She needs to shield/heal someone else to trigger the on-hit boost of ardent for herself, and she's often in a solo lane, and people generally prefer being the full AS late game hypercarry rather than supportive (as various more directly on-hit or AP items are going to be better boosts to her dps).
It's a good item on a support kayle, and is key in certain boosting strategies where theres a reliable yi or similar she can focus on buffing, but people tend to focus on their own potential to solo carry and a supportive item isn't what people are inclined towards there (and if someone was keen to support the team, theres probably better champion picks anyway).
Her heal is on a pretty high cooldown until it's maxed.
Kayle is better off buying damage items, she is a hyper carry after all.
Meh, if you're going to buy Ardent on Kayle, you might as well build full support items. Kayle's primary function is to serve as DPS, buying Ardent would mess that up, especially in a splitpush scenario.
