This isn't a rule or anything along those line, its just comes more from experience in hosting LAN's for over a decade.
When your hosting a PC or Console gaming event, it can actually be quite difficult to organise and run. By running "older" games, we solve a couple problems
Price.People need to own the game to play it. We don't actively hunt down non-legit games, but it does make it easier for everyone if the games we play are cheaply available.
Compatibility. The games need to run on a range of hardware. While we defiantly have our fair share of monster PC rigs, some people don't have the luxury to have the latest graphics card, or even a gaming PC at all. Its not rare at all to see some attendees with laptops.
Setup. To save people having to install new games everytime they come to the LAN, we have a list of core games that work well for the event. Newer games have a habit of being updated constantly, which can be an issue at a LAN event.
Acceptance. When running a game for the whole LAN to enjoy, its important that people actually like the game. Over time we have developed a list of games that people have enjoyed as a whole, and as a natural effect, that list of games is of more established games.
Even with these points laid out, we don't avoid new games, and have been known to try new things from time to time. Also, with the latest, biggest games, you'll usually find people playing them around the LAN outside of the organised games, so feel free to ask them to join in.