They've said nothing
The last time this topic was discussed was during Pathfinder's initial development, when people discussed why druids had some abilities and not others like in Pathfinder's antecedent dnd-3.5e where druids had many other now-absent abilities.
Searching through the threads, I noticed that even when a developer answered questions, this specific topic wasn't addressed. But, to be fair, I never encountered this exact question on my research.
All we really get by way of insight is a post from designer Jason Bulmahn about the lack of a changelog during the development process:
I would really like to help you guys out, but the changes were quite numerous and done over about a three week period, based off notes, conversations, and extensive lists pulled from these forums. If I want to keep the Beta rolling along, I am afraid that I just do not have the time to pull together this list.
My apologies to your thirsty ink cartridges.
A few days later in the same thread, a user noted a change in the druid spell list:
Spells (pg 99-114): The spells Beast Shape I+, Elemental Body I+ and Plant Shape I+ have been removed from the druid spell list. The only polymorph spells that druids now get are Animal Shapes, Baleful Polymorph and Shapechange.
(Emphasis mine.) And that's all we know.