I also figured I'd put in some info that have been said by Maro and Roy Graham (the new story leader) as the Doylist reasonings for the mass desparking from design (a bit of development) and creative ;
-As cards planeswalkers are shallow in design, often limited to sorcery based activated abilities and need to have ~ three of these abilities that all fit in the card box. Also add in that many planeswalkers are meant to have "gimmicks" and in general feel in flavor to the character as well as trying to balance them for what format they made for has lead the space of good planeswalker cards tricky.
-Development would also want planeswalkers in standard to keep a rough color balance (which often could be use to predict the color(s) of walkers in upcoming sets). This would mean deign and creative sometimes where locked into having to have walkers in certain colors/color pairs for sets. For design this lead to some stuff such as why we didn't get a
BR in a standard for years in-between Sarkhan the Mad and Angrath and creative this why often characters where pushed into new colors or had very random feeling characters pop up in certain sets.
-Planeswalker cards slots are limited meaning they can only feature a certain amount of characters in the cards vs legendary creatures which sets have a lot more open slots for.
-Many planeswalkers characters where made as "extra special" faces of their planes with not very much reason for leaving and/or story and character arcs pretty much where closed and desparking them is a way to let them "retire" without killing off the character in the story and letting them still have stories and being faces for the planes when we visit. To this end creative wants to have the remains planeswalkers have more motivations for going to different planes.
-All levels wanted planeswalkers to start feeling more special and this has been done via limiting the number of them we see in sets and who are left in the lore.
EDIT:
- Even after the mending planeswalkers ability to planes walk caused some tricky-ness with writing as it meant each conflict had to address why the planeswalkers in the storyline where sticking around.
“There are no weak Jews. I am descended from those who wrestle angels and kill giants. We were chosen by God. You were chosen by a pathetic little man who can't seem to grow a full mustache"
