There are lots of reasons to pick Google over Yahoo/Bing, the most important is that it has the highest consumer usage. And, you can see from this chart, most advertisers agree that Google is the first place to spend their money.
The other challenge, besides limited ad budgets, is the overhead it takes to manage and track local campaigns. That overhead doubles when you add a second search engine and that hurts your ROI. Most marketers decide that Google is the most popular and there is plenty of inventory available, so cover that base first. We agree.