Why is LibreOffice default instead of OpenOffice.org
Assuming you read What is the difference between OpenOffice and the newly created LibreOffice? (I was going to mark it as duplicate but your question is more of a why Ubuntu and others still use LibreOffice) the differences right now and possibly for the future are:
LibreOffice Development is several times faster than OpenOffice which helps users and distros get better performance, stability and features ready for when a new distro version comes out.
Many of the changes users have asked for years with OpenOffice were implemented in less than a year in LibreOffice (Many types of formats for example)
Staying with LibreOffice gave some guarantee to all Distros that the next version would not magically change licence and try to make them pay for it.
When the Apache Foundation took over, it was already too late. LibreOffice had many features, updates and commits made that were much better than OpenOffice and it had a programming rhythm that was and is faster than the one OpenOffice has.
Trying to work on OpenOffice, knowing the above points is leaving many of the things LibreOffice has done and working twice as much to implement some of them back to OpenOffice.
Now for my personal opinions over this based on experiences with both and since the creation of LibreOffice:
The move from OpenOffice to LibreOffice was basically (And this is my personal opinion) because Oracle is simply a not so friendly open-source type company that since the moment they touched everything Sun had, it became to some degrees, unfriendly for the open source user (VirtualBox, MySQL, OpenOffice and Java to name a few). They started changing many things. From VirtualBox not offering USB support (Yes I still remember when VirtualBox was installed and you had USB 2.0 support from the start) without the extension, to MySQL changing parts of its Licence, to OpenOffice changing many development aspects and where its heading was. This is the reason many of its developers simply left and created the documentfoundation.org because they believe in the open source idea of sharing without limiting and knowledge before money.
LibreOffice covers many file types that I needed for many years and OpenOffice did not offer. I for one made about 50 students join Ubuntu just because LibreOffice not only supported older file types, but had better support for doc and docx documents.
The memory footprint of LibreOffice is less than OpenOffice, so it helps computers with limited resources.
Since the development is faster, many of us can pinpoint a bug and most of them are targeted very fast and solved very quickly. Not only that but as you can see there is even a contest to find Bugs and win a TShirt.
I personally have seen more bugs and fixes applied to LibreOffice in 6 months than 3 years in OpenOffice. you can see the development of Apache OpenOffice vs LibreOffice. As you can see LibreOffice has almost 20 times faster development than OpenOffice right now. That gives anybody an idea to which one to stick.
The team behind Apache OpenOffice is not in a hurry and although they are an excellent group and Apache, well, what can I say of this amazing foundation that you have not heard, comparing speeds of development, I would not be amazed if they left OpenOffice to die its last days.