The two RR MT-30 gas turbines in the DDG-1000 have the power requirements needed for the future rail gun and laser weapons. Could the Burkes realistically be upgraded with the sufficient power needed for these future weapons while still maintaining hotel and propulsion loads? Also, the HVP tech velocity pales in comparison with the railgun.
AFAIK lasers and railguns weren't a consideration when DDG-1000 was being conceived. The focus was on AGS. If HVP had been foreseen at the time then IMO justifying the expense of a new class of DDG to fire PGMs from ship cannon would have been problematic.