Why do I owe more taxes though I made less money?

It could be the credit. If each of you has a W-4 saying "Married", then each of you would have had your withholding reduced by $800 (the credit for a married couple). That is $1600 less withholding, but you only get $800 as a couple. Filing separately you still only get $800. If you were single you would each get $400. And by the way, you aren't paying $1500 more in taxes. You probably are paying less, but had much less withheld, so didn't pay over the course of the year.