I have spent a good amount of time putting thought into why some ranged specs / classes are given mobility, while others are not, or at the very least, reduced damage while moving.
The fundamental difference (which needs to be understood, imo) between a melee class and a ranged class in terms of damage while moving comes down to "time on target". A melee class, such as a DK, when forced off of a boss, often have no offensive capability. At best, they have a severely diminished ability to do damage to a boss. Since the area that a melee is capable of doing "full" damage to a boss is roughly 1/64th of the area that a ranged is capable of doing "full" damage to a boss (1/128th if you factor in positioning of the melee "behind" the boss), we can see the issue in terms of how encounters are designed. In order for the melee to be competitive with their ranged counterparts, there needs to be some way to balance the difference in movable area. This is generally accounted for with mechanics that force the ranged, or everyone, to move. A basic example is Attenuation. Everyone needs to move. This is "supposed" to have more of an impact on the ranged as a way to balance times when melee are forced out of their tiny area where they can do "full" dps. The trade off for a melee forced out of range is that if the time is short enough, they regen resources to unleash attacks when they do get back on the boss, negating some of the time spent doing 0 damage.
We currently have 6 classes that are capable of dpsing from range. Between them, they account for 12 (13 if you count holy priests) of the possible 34 specs.
Of those 6 classes, only 1 has the ability to retain 100% damage while on the move (hunters) For now, I will leave hunters out of this discussion as frankly, the system they are using for mobility makes zero sense to me.
Of the remaining 5 (looking at 5.2 here for clarification), only mages have the ability to cast a number of instant-cast spells to fulfill a sort of "moving rotation" without investing in a glyph or a talent. FYI - A mage's damage drops to roughly 15-40% of normal while moving (except for fire... who is the odd duck here - they retain about 65-75% of their damage).
We're now down to 4 classes. 1 of these can use a glyph that allows them to cast their "filler" spell while on the move (shaman). And 1 class has the ability to talent into the ability to cast everything on the move (warlocks). Both of these options have a drawback. Personally, I feel the drawback on either is fairly small, but I am sure that someone will disagree with me. To each their own.
This leaves us with just 2 classes that have no real "on the move" rotation. Nor do either of these classes have the ability to become more reliably mobile through the use of a glyph or a talent. I have begun to really wonder why these 2 classes were singled out for this. Then it occurred to me - both of these classes have a spammable dot. While that is nothing to brag about, as warlocks and mages (who don't take frost bomb) both have a spammable dot, the difference is that those dots don't have the direct damage component to them that moonfire / SW:P do.
So it occurs to me that Blizzard looked at the ranged classes, said "Ok, Priests and Moonkins have a dot that does damage. They can spam that while on the move, so it will make them comparable to a melee forced out of range. Seems good" They then looked at Warlocks and Mages and said "Well, mages have a number of instant cast spells. They can use those while on the move. While they will be better than a moonkin on the run, they'll still be pretty bad. So we're good there. But Warlocks... what do do with them... " Thus, KJC was born as a talent - a way for a warlock to still deliver some damage from range, with penalties to discourage extended casting while moving. The original incarnation of KJC was much steeper in terms of cost than the current one.
Blizzard then turned their attention to Ele Shaman and said "Well, what have we got for them? We can have them spam flame shock? Nope - 5 sec CD, that won't do. Its just too little. Well, we gave them that cast LB on the move glyph in Cata, want to let them keep that? Sure! LB is only a small portion of their overall damage, well, small enough that it will be ok" and tada! Shaman are now in line with your average melee when forced off the boss"
TL;DR So, in looking for an answer as to why some classes have more mobility than others, it seems to me that the original goal was to make movement for ranged approximately as punishing as leaving their target is for melee.
Thoughts? Alternative ideas?