If Legalizing unauthorized immigrants would help our economy, why are Republicans against Immigration Reform?

Because they truly wish to exclude them. Giving "them" a pathway to citizenship would mean that "they" would become "us", and "we'd" have to extend to "them" the same rights and benefits "we" enjoy as citizens. And the conservatives, a) don't want to pay; and b) don't want to pay for non-white, non-born citizens' needs... just like they don't want to pay now for their human needs.