Maybe the area you think might have been a better choice did not have good drainage - how did it Perc?????

This is often code, water table, a million things. Consult the town you live in. There could have bee MANY reasons. Consult the original builder. Nobody here can advise, that's for certain.

Stax and Jan Moyer are correct - it must have been done for a reason. Perhaps the soil in the flat area isn't good for drainage. Is there an existing soils report? As Jan said, consult the appropriate department in your town or county jurisdiction. Moving your field lines might require new perc tests and a larger drain field. Good luck!