Why is a LTS release not using the Extended Support Version of Thunderbird?

Because we use Firefox 11 instead of ESR and most of the same reasons apply to Thunderbird as Firefox for using the rapid release version. The decision to not use ESR by default in the LTS was surprising to me, so I asked on the mailinglists (you can find the thread here: https://lists.ubuntu.com/archives/ubuntu-desktop/2012-February/003672.html) and Jason Warner from Canonical answered:

Firefox adopted a rapid release model for various reasons, but among them was that they needed the browser to keep up with the pace of innovation on the internet. Ubuntu needs to be out in front of these things and be pushing the very edge of what is possible, particularly in the browser. I do not think we can ship a browser that will lag by 12 months in any sense; the risks too far outweigh the rewards.

I'm afraid that even a year lag (ESR update period) would put Ubuntu at severe disadvantage to other platforms. Imagine a world where G+ or Facebook or some new whizbang product didn't work on Ubuntu because the browser shipped didn't support some new technology/javascript engine/platform component. That is neither something we want nor can afford. We have to be better, we have to be faster and we have to be braver.

Anyway, you'd like to install Thunderbird ESR, you can donwload it here: http://mozilla.org/en-US/thunderbird/organizations/all-esr.html You simply need to extract it into your home folder and add a script called ~/bin/thunderbird to launch ~/thunderbird_folder/thunderbird $@ every time your user calls thunderbird

You can also see this blog post from Chris Coulson on why the ESR was not selected.