They carryforward and attach to the property(ies) received in exchange. It is because the property was exchanged in a nonrecognition transaction, not disposed of in a fully taxable transaction. It's under sec 469, sorry don't have the exact reference. It's the same section that says the PALs can't be used in other nonrecognition transactions such as 351 and 721 transfers and when a passive activity property is sold as an installment sale (PAL allowed as gain is recognized in that case).