Why did they cancel Mars Mission? Does anybody know why LEGO canceled the Mars Mission line? It was so good and I almost had everything! I don't wanna buy off eBay because that can get expensive. I liked it a lot!
In this forum, I found a reply from the Lego Group in response to an email asking a similar question but related to the Mars Mission games:

Thank you for contacting us and for your interest in LEGO® brand products.
Unfortunately, because The LEGO Mars Mission line you inquired about has been discontinued from our current product line, we do not intend to create new games based on this theme. Each year, several items are discontinued and new ones added. By doing this, we provide a product line with more variety and continuing challenges to our LEGO builders. You may like a game based on our Space Police theme called Galactic Pursuit, http://spacepolice.lego.com/en-US/games/default.aspx. You may also like a multiplayer interactive game named LEGO Universe that will be released this upcoming summer. To preview this new game, you can visit LEGOUniverse.com. I hope this information helps!
Thank you again for contacting us.
--LEGO Direct Consumer Services