Title is wrong... Android 2.3 is not the most popular, its just most the largest market share or most used.
1- Google never made 3.x for phones, it was for tablets only... hence a bit of a delay in phone OS development.
2 - The issues between Phonemakersphone companies makes upgrades a bit tricky... But the blame still goes to ALL 3 (example): Google>Samsung>at&t. And since its been about 2 years since Android 2.1~2.3, our phones are just now ending their lifespan and contracts.
In the past 4 weeks, Myself and 5 other friends/etc have upgraded our 2 year-old phones. Those on WP7 have gone to ICS as well as us 2.x users going to 4.0/4.1. Watch 2.3 lose 5~7% market share a month as those devices are recycled/retired. Since my Galaxy S1 still works (mostly) - Its been turned into a mini-WIFI tablet/camera for the kid. In 6 months, it should look a lot better for 4.x. Of course WP8 will starting eating its 1% share.
3 - if Android 4.X was available for anyone, more people would be on it by now. Apple only makes a few kinds of phones, with 1~2 models a year... easy for them to upgrade the OS. Unlike Android with about 30~40 phones a year.
4 - Until recently with today's new 1280x720 display phones with 4.0, there wasn't much reason to upgrade.