Why are some PWs so different from their paper forms?

This thread is mostly a question of curiosity: Why is it that when you import planeswalkers from paper Magic to MtGPQ, some of them have their abilities greatly changed?

Now I know some aspects in paper Magic cannot be translated as is to PQ, but some planeswalkers have their abilities so radically different between paper and PQ forms that it leaves me flabbergasted as to why you make such changes.

I mean, for example, I can understand why Liliana, Defiant Necromancer has Grave Mist ability; her paper forms has two abilities that both handle reanimation, and thus for you to add in something different is a welcome change (given that Grave Mist in a way feeds her third ability). I also applaud on how you change Jace, Telepath Unbound's milling ability into mana destruction, given that the recycling deck aspect of PQ makes milling probably impossible to implement. Gideon, Battle-Forged, have changes that are mostly acceptable: Giving Vigilance (and later Defender) mimics his paper form's ability to force people to attack the wrong target, although his anthem ability (second one) is a little questionable.

But then we have Garruk Wildspeaker. His paper form has abilities that give you some mana and pump your army. In PQ, somehow all he does is make vanilla tokens of varying sizes. Why is this so? Sorin, Grim Nemesis also; Taking your latest patch update into account, instead of drawing card to hurt opponent and directly draining life from an enemy (a la Lightning Helix), you somehow turn him into a full-blown healer/survivalist with no ability that directly harm an opponent. Why? (at least his third ability, strengthening your army by gaining life, is related to his paper form's ultimate of making an army of vampires whose size depend on players' life totals, so for me that's acceptable).

Sure, changes are sometimes fine when it comes to different media, but given that for most planeswalkers you either manage to copy the abilities verbatim (like Ob Nixilis, Reignited and Kiora, Master of the Depths) or replace game-incompatible abilities with something that has similar feel (like Jace, Telepath Unbound and Tezzeret, The Seeker), some of these drastic changes feel rather odd.

Care to explain?


PS: And an unrelated question: Where is Chandra, Flamecaller and Nissa, Voice of Zendikar? We're already in SOI and we still haven't seen them...