The reasons are many and very complicated, but here are the cliff notes version from Teodoro Agoncillo's "A History of the Filipino People":

• United States politics was not a unified body with regards to the issue of Imperialism. McKinley for instance was a strong supporter of imperialism, but Wilson fifteen years later espoused self-determination in Europe and that the population's wishes be taken into account in the colonies.
• At the time the Philippines became an American colony, the imperialist supporters were ascendant, and the plan was to make the Philippines as a bastion for American naval presence in East Asia.
• The problem is that the United States had made a deal with Spain, which at the time had basically lost control of the colony already due to a local rebellion under Aguinaldo. The Aguinaldo government, which had declared independence, understandably refused to recognize the transfer of the "colony" to the Americans. Worse, and with tragic consequences for the Muslims of Sulu, the Spanish claimed that Sulu was part of the Philippines despite never actually conquering the place. Bloodshed very quickly followed as the surprised Americans found themselves fighting elements of the Aguinaldo government in the north/center of the country and the Muslims in the south; both of whom did not accept their deal with the Spanish.
• Multiple years of bloody insurgency followed, which was reported in the press. This rapidly dampened the imperialist ascendancy, with plans for using the Philippines as an East Asia base rapidly being deemed too costly as America discovered the real cost of running a major colony.
• The options that emerged essentially boiled down to two possibilities: Independence, or full statehood. I'll discuss the statehood thing first.
• It was determined that given the size and population of the Philippine islands - which had 10 million people living in it at the time - it simply could not be maintained as a dependency like Puerto Rico. If it was going to be part of America, it HAD to be a state or even multiple states. This was not very popular with American politicians, as it would very greatly change the balance of power in the Congress and Senate.
• The resolution instead was the 1916 Jones Law, which proposed full independence albeit without a definite timetable yet (the timetable would be defined by the Tyding-McDuffie act in 1934). This gained support in the US in large part because the Filipino resident commissioners supported independence, and that support for statehood also proved to be weak among the local Filipino population.

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Finally, I would note that America was receiving huge numbers of immigrants at the time, so the idea of giving "foreigners" power in Congress was not the issue. The real issue was how it would upset Congress and the Senate due to the addition of many, many more seats to both Houses.

The chances of the Philippines becoming a state or multiple states of the Union moreover were not zero - Hawaii was 60% "Asian" when finally admitted to the Union. However with the Manila representatives insisting on independence the American government would have to actively insist and appear imperialist to push for statehood. That simply wasn't going to happen after the Filipino-American war resulted in most of America flipping back to anti-imperialism.