Ambassador posts, other than maybe five to ten per country*, are given to moneyed donors and upper crust wannabe campaign sloggers. Obama's appointees were pretty much vetted through the DNC's list of who we owe favors to. This has been going on for hundreds of years. With electronic communication, the ambassador's post is superfluous but it gives the host country someone to invite for ribbon-cutting ceremonies.
*This differs depending on what country you're in. The list of important embassies would be: your closest neighbors, G8 and megapowers, the places you want to do a lot of business with.
All the rest are paid vacations. To the host countries, it's different, particularly in a backwater like Thailand. They actually think the US Ambassador is someone important on the world stage. Plus, he's particularly useful to have someone to rally against when you need to stage a "spontaneous" demonstration.