WASHINGTON — Federal officials often say that health insurance will cost consumers less than expected under President Obama's health care law. But they rarely mention one big reason: many insurers are significantly limiting the choices of doctors and hospitals available to consumers. From California to Illinois to New Hampshire, and in many states in between, insurers are driving down premiums by restricting the number of providers who will treat patients in their new health plans. When insurance marketplaces open on Oct. 1, most of those shopping for coverage will be low- and moderate-income people for whom price is paramount.