The event had been set for 18 April and would have seen Ospreys play Dragons and Scarlets take on Cardiff Blues at the Principality Stadium.
The WRU had hoped to reschedule Judgement Day after the Pro14 season restarts on 22 August but social distancing rules make it unfeasible.
Those who had bought tickets have the option of refunds or credit notes.
The loss of the Welsh Pro14 double-headers at the Principality Stadium is another financial blow to the WRU, which has already said it will seek a loan to help it through the financial effects of coronavirus.
"The prospect of hosting the Judgement Day event at Principality Stadium as a way of restarting the season after lockdown was an exciting one, but current restrictions and predicted return to rugby protocols no longer make this a realistic option," said a WRU ticket office spokesperson.
"We are still optimistic that when professional rugby returns there will be a way for spectators to return at a similar time, or soon after.
"But the safety of everyone involved will be of paramount importance and all the indications are that a Judgement Day event for the 2019/2020 season fixture list is too ambitious at this stage.
"Of course anyone who would like their money back for the tickets they have purchased will be processed as swiftly as possible.
"We have also come up with a creative way for fans to support their region at a time when they perhaps need it most, by giving the option to donate funds to the Ospreys, Scarlets, Dragons or Cardiff Blues instead of claiming reimbursement."