Tier list!

[Cynric]

snip

Bard is open to heavy counter play through CC and cleanses, has low CC itself, is reliant on buff timers, can only buff ninja /healer damage with foes due to forced comps, is rooted by cast times for highest burst, has a ranged penalty on damage, only one self heal, and can be shut down easily against capable players.

As matches get shorter the ability to regen mana will also become less useful. While regening mana you also incur a further damage penalty. Yes your burst is decent, but that's all you really have. Capable healers, DPS, and tanks can heal through, cleanse or shut down bard burst using CC.

Stone skin isn't the only thing white mage has. White mage has multiple forms of CC on differing diminishing returns, strong heals made better through the interruption change in patch 3.22, multiple forms of regen to help handle split damage, and great mp sustainability without a BRD/MCH.

Any DPS can burst a healer down through stoneskin, depending on the relative skill levels of both the healer and the DPS. However how often can said DPS do that? How much utility do they bring? How much CC? How badly are they countered through normal means like purify?

Bards burst availability, level of CC, and actually usable utility just isn't that great.

I'd also like to note that Wardens fades upon anything detrimental including DoTs. One DoT and a healer could be slept again, You could remove a Dragon Kick or a Storms eye/Dancing Edge, they could be slept and binded and you could just remove the bind.

While wardens has good potential its general variation among the debuffs weakens it, most debuffs and CC can also be re used very quickly. You'd have to babysit your healer to get real use out of it honestly.