Why distributed AG is preferred to normal AG for cross datacenter failover scenario?
what are benefits of using distributed AG compared to normal AG?


Complete isolation of your clusters is the only benefit.  


  Can we just stretch WSFC across data center and make this happen with normal AG?


Yes, and you normally should do just that.  Stretch clusters are not complicated, and modern versions of Windows make them downright simple.  IE the cluster node voting configuration and quorum is pretty much automatic.  And you can even implement automatic cross-datacenter fail-over with Cloud Witness.