Why does bash bundles readline instead of use a shared one?

[Garreau, Alexandre]

Hi,

Today, searching in my distribution (debian) for which packages depended
on readline, I learnt that bash didn’t link with readline, but bundled
its own version present in its sources tree.

Why so? is it to gain performance? is the memory waste minimal or even
negative due to some optimization? is it because distributions may
find difficult to embed an instance of a recent-enough readline? doesn’t
this add complexity to manage synchronization between bash readline and
upstream one? I’m curious about this…