If you ask why they pick Forza>Fallout, it's because 2015 is 10th year anniversary of Forza series.
Fallout? maybe 2 years later when Obsidian make a Fallout:New something.
Its painfully obvious that people arent giving a shit about Forza 6, the game is currently at #116 on the Amazon Top 100 (Never cracked the Top 20), and the bundle is sitting at #169 (Never cracked the Top 20). The game simply failed, flopped.
There is no way that a series that is annualized, no matter that its 1st party, has a preference when it comes to bundling, specially with a game of the caliber of Fallout.
Microsoft had the neccesary time to realize that Forza 6 flopped, and bet on arguably one of the biggest games of this holidays. But hey...this is just my honest opinion.