Parties originate from the Users
Launch Parties are pushed by the users not any corporate entity promoting the operating system. It is truly mind boggling* how a corporate entity is trying to get people to celebrate in their own homes something which they may not feel attached to. And how can you be attached to any software that asks you to fork over lump sums of money and in many cases treats any bug reports you send as misuse of their software (another mind boggling issue that is a subject of an entirely separate post). Emotional attachment to software is important, normally this was only experienced by the developers. However, free open source softwares have unveiled that this attachment can extend to the users. A discovery that was not observed with years of proprietary software.
Ubuntu is free (as in costs $0)
This is a great reason for the success of any launch party. Why would someone be interested in having a launch party and showing off their cool new OS in a specific private event, if they cannot even distribute copies of it to their friends and families. Richard Stallman has great pieces about how proprietery licencses destroy the fabric of society. A clear example is if you show off a new piece of software to a friend of yours, and they like it so much that they want a copy of it. If it is proprietary software you may be obliged due to licensing to not give them a copy of the software which you paid hard earned cash to own. This pushes the meaning of ownership far beyond its real meaning (that is a subject of many papers and articles, please see http://www.fsf.org/ for more readings).
Ubuntu is free (as in FREEDOM)
This is by far the most important reason why there are so many release parties taking place across the world. Everybody can contribute to Linux and any of the free distributions based on it. Therefore, there is a vast amount of developers working on the project. moreover, these developers do not get paid (arguably this creates much higher quality software than paid development) and even if they do, they are working on the project because they are emotionally attached to the software. This causes many flame-wars (http://en.wikipedia.org/wiki/Flaming_%28Internet%29) which are in essence an embodiment of emotional attachment to software. People feel strongly about the software they support and thus promote it strongly. I have yet to see something as big as a flame-war erupt due to proprietary software preferences or even hardware, Free and Open Source Software have really changed the way people feel about applicable software.
Every six months, namely in the months of April (04) and October (10) of every year a new release of ubuntu is leashed on the market. Every fourth release, including the one coming up in April 2010 (4/2010 or Ubuntu 10.04 “Lucid Lynx”) is an LTS release and is supported for 5 years (on the server), which makes them ideal for corporate and mass deployment. For more info on Ubuntu LTS releases visit https://wiki.ubuntu.com/LTS.
Having regular releases warrants for more release parties and a greater awareness of what has changed in the operating system. Many new users are turning to Ubuntu (or Linux in general) every day. Recently two of my friends have started with WUBI, and in no time (< 1 week) one of them is formatting his drive to install an Alpha 6 version of Karmic Koala (9.10) which due to be released end of Oct this year.
Launch Parties are not the only time were awareness spreads about free non-proprietery operating systems, there is another invention known a an install fest. These are events where members of the general public whether members of a User Group (e.g. www.nslug.ns.ca), or not, gather around to install a copy of the operating system and help each other out with setting up and customising their personal machines.
Note, you may replace Ubuntu with any Free GNU/Linux variant, and the arguments still apply. The points above contribute to the success of launch parties, and they are apparent that they are not available with any Proprietary OS. Although I have mentioned Ubuntu quite a bit in my post, I have recently tried out a flock of Linucies, and think I will be settling with Debian. I am currently running Debian Squeeze and it is a treat.
* mind boggling is a trademark of Toucan (tdas), a good friend of mine.