Charm power curveCharm is an ability that requires some percentage of the caster's concentration. This means some percentage of the caster's time (mana regen). Since mobs increase on an X^2 power curve, and so does player power, the charm ability, to stay balanced, has to increase in mana cost such that the amount of caster time (mana regen) it consumes is constant.
Bards, for a given ability, always have the same time cost: 1/3. This is why bard songs are meant to always be useful to the bard, because the cost doesn't decrease like caster spells. One charm song is all that is needed for bards, it just needs to have the charm cap scale with bard level.
Charm level capOriginally there were no charm caps, but they were intended to be there. They were finally added in a quick fashion because they were badly needed. This quick fix involved making all charms cap at the level of the charm spell, +10. These days there are some spells that have exceptions to this rule, starting with the level 49 charm spells, but originally it was this way for every charm spell.
This method caused problems. For example the shaman druidic charms, which are gained much later by shamans than druids, could charm anything that was up to 10 levels higher than the level the SHAMAN got the spell. So a shaman could use a given (cheap) charm spell and charm a blue, or even a red, while a druid with the same charm spell could only charm some greens. This was later fixed, for those particular spells.
So what was missed in all of these changes? Bards. Even though charm is a core bard ability (just like warriors in fantasy worlds are tanks, bards are charmers: the Pied Piper, pan pipe playing Satyrs, snake charmers, etc.), the impact on bards wasn't considered.
Bard impactBard songs are supposed to scale with the bard, but since bards weren't thought of, the bard charm song was capped at level+10=37. There was only one bard song for bards because bards were meant to only need one, yet due to the oversight it only worked for 10 levels.
But there was a problem with bard charm, that had cropped up while all of this was happening, and the designers were not happy.
Charm kitingCharm kiting is when a bard charms one mob, say, a hill giant, and has it attack another. Pretty soon the pet kills the target, and the bard moves on to the next. The bard then has a hurt HG, and either chants it to death or has a new HG kill it, getting fast xp from the prior pet. The bard experiences little risk while all this is going on. Bards using this technique frequently trained other people (unintentionally), and other people soloing the same mobs saw bards killing more mobs than they were. People were not happy, thus the designers were not happy.
So how did charm kiting affect the whole charm cap issue? Well charm kiting was unforseen and considered evil. Nobody was interested in increasing bard charm power when it was obviously too good in this one situation. Eventually level 39 charm was added as a fix.
The new level 39 charmThe level 39 charm fixed charm kiting by adding a mana cost. So the bard could charm kite, but not forever. GZ was always a big proponent of kiting, and I am guessing this was the end solution: kiting was allowed, but limited in duration.
I don't know the whole internal story behind the 39 charm. I don't know if it was meant to scale up with bard level, or had a charm cap equal to the highest kunark charms, or what. It was added fairly close to the kunark release, after the kunark spells were done. It worked on mobs up to level 55, and maybe higher.
All it took was a few gods getting charmed by bards for the nerf hammer to come down, and bard charm was changed to cap at 51. 51 was definitely the best cap pre-kunark, because so many encounters were designed assuming that cap existed. Having a higher cap made some of the toughest encounters at the time (PoS, etc.) easy.
Of course now, enchanters can charm level mobs over level 51 easily.
KunarkKunark came out and there was no improved bard charm. I'm guessing that the 39 charm was meant to be the charm for bards, and was meant to scale up in level cap as the bard gained levels. All bard songs are supposed to work this way, and there should be no need to ever improve an old bard song. But the code didn't exist. And then GZ left.
Prior to kunark, bards were at minimum the equal of enchanters in charm, and in most situations, better. The bard charm cap is 51 at level 39, 51 at level 50...and still 51 at level 60.