So from what I understand...
Supporting Return
+Apparently Popular in Japan.
+Apparently liked by Sakurai/GameFreak.
+Original 12. Inclusion is basically a tradition.
+Low priority in Brawl, but considered a "lock" due to eventual inclusion.
+Known by casuals for it's Smash Bros appearance. Decent sized fanbase.
+Common Pokemon in X/Y, received attention with new typing.
+Among the easiest characters to program, if not the easiest.
+Likely higher priority than clones/semi-clones like Wolf and Lucas.
+Unique moveset and playstyle.
Against Return
-Relevancy/Recency issues.
-Almost cut in Brawl. Possibly low priority again.
-May be considered "dated."
-Brawl was developed like it was the "last game in the series." Jigglypuff's inclusion could have been added in merely to complete the original 12.
-Generally disliked by countries other than Japan.
-Set deadline for SSB4's release. Brawl was delayed numerous times likely to include more characters.
-Possibility that a newcomer will inherit moveset traits and "replace" Jigglypuff.
Obviously the pros outweigh the cons.
But there's always that...
That unspeakable...