Presumably because the old API was broken/lacking/etc. Calling the resulting porting work "unnecessary" without even considering why the API might have been changed is not fair.
The pidgin project is 25 years old, and libpurple is 17 years old. It's entirely fair that a rework is required, and much appreciated that this work is continuing.
The pidgin project is 25 years old, and libpurple is 17 years old. It's entirely fair that a rework is required, and much appreciated that this work is continuing.