Why I was born with brown teeth?

Which ones. Your baby teeth can be stained by infection during pregnancy or some drugs. Some of the baby vitamin products stain the teeth if they come in direct contact during dosing.Your adult teeth are developed later & pregnancy would not effect them, so you wouldn't be born with them brown.
Tetracycline. Often if tetracycline is taken while the teeth are developing. The teeth will appear brown. You have some options to whiten your teeth, like veneers. Consult with your dentist.
Brown teeth. The tips of your upper dental incisors and lower 6 year molars form before you are born. The rest of the tooth crown and roots form after birth. There are many intrinsic and extrinsic reasons for tooth discoloration. Your Dentist will be able to tell the difference between surface stain, genetic abnormalities, and damage to the enamel, and then recommend to you the appropriate treatment.