Because unlike the British written language, the American written language has actually taken some steps to make words be spelled as they're typically pronounced, such as "color" which doesn't have a U sound in it at all (if it did, it would sound like "cuh-LOHR"), or "recognize" which our friends across the pond insist on spelling with an S even though there's no S sound there. It's linguistic evolution, and it's arguably the right thing to do.
When will we as English-speakers finally correct our ridiculous many ways of pronouncing the word suffix "-ough" in some manner?