Because probably they thought after long and very thoughtful brainstorm that providing info like this:
http://wiki.maemo.org/Maemo_5/PR1.2

Is waste of time so let´s just report hmmm: Only bugs that were in public bugzilla and let´s write in blog about many fixes and tweaks. Let´s not give too much information to users so that they do not get baffled.
http://wiki.maemo.org/Maemo_5/PR1.3

"hundreds of tweak and fixes" ...yeah sure right. Show change-log to us.
http://conversations.nokia.com/2010/...ate-available/

.edit
Is it really so that Only Andre Klapper is writing those? Surely Nokia has it´s own inner change-log that includes stuff that is fixed and can be seen in e.g. modest&hildon-desktop git repositories.