If most Americans are descended from Britons, how come they don't have the same accents?

Since most Americans are not descended from Britons, this question is essentially moot. Only about 9% or so of Americans are directly of English descent.

Immigrants from many countries came here, to what was once a British colony, and adapted their native tongues to what the Brits had established as the primary language; the results of the uneven mixing are regional dialects. 

Because they are influenced by the accents of people around them, as well.  Thus the accent of (say) Louisiana was influenced by the French ... And the accent of (say) Minnesota was influence by the Scandinavians ... And the accent of (say) Texas was influenced by the Spaniards.

Because here in tx we speak tex mex half english and some spanish and some british. So yee haw