FTC has questions for Google about AdMob acquisition

Google announced their plans to acquire AdMob for $750 million, one of the most used mobile advertisement companies, last month. AdMob basically makes it possible for mobile application developers to monetize free apps -- and at the same time give advertisers a window into mobile devices. Even though Google didn't see any reason there would be issues regarding the acquisition, the FTC is asking for more information before they let the deal go through.

Google's argument is that the mobile ad space is extremely competitive already -- and so acquiring one of the many shouldn't be cause for alarm. Is that really how it works though? I'd think that since the market is so fragmented right now that an acquisition of this magnitude could put smaller companies like AdMob out of business. Basically Google will bring to the table an enormous quantity of advertisers currently using the AdWords network -- something other companies like AdMob simply don't, and will never have.

Why would a developer choose to use an ad network whose advertiser base is dwarfed by another network?