What would cause low water pressure in the kitchen sink and the sprayer still have full pressure?

Try to remove the aerator from the end of the faucet and check it for any debris, this is most likely what the problem is. If that doesn't work try to clean out the sprayer diverter, see this link for how to do that:

How to Troubleshoot Common Kitchen Faucet Problems - Associated Content

Or wait for some of the great plumbers that are here to answer you, they may have better ideas than me.

I agree with amricca, ts likely a clogged aerator if your sprayer is working good.