I would guess because the developers want to target as wide variety of devices as possible. The amount of devices running 2.x are diminishing rapidly now of days. I don't know that it's really the wisest thing to attempt to support that variety of target OSes for a small dev. Besides, if you restrict the lowest platform available, in a small way, you also restrict the minimum hardware your game will run on, and in a sense, give you tighter control.
According to Google, only 13.6% of devices run Gingerbread. Only 0.7% run Froyo. More interestingly, only 10.6% only run ICS (4.0). You can see the breakdown here https://developer.android.com/about/dashboards/index.html?utm_source=ausdroid.net
More interestingly, as of November of last year, Samsung held 61% of global Android market share. I've heard that Samsung now holds closer to 80% market share though I don't have facts to back that up. http://bgr.com/2013/11/08/samsung-android-devices-market-share/
Given those facts, I'd say that it would probably be safe to limit the target platform to 4.1. Even generic knock-off Android tablets ship with 4.1 at minimum anymore. Most Samsung phones run 4.1 or above anymore.