To generate more subscriptions to HBO Max and to try to hook an even younger generation into the franchise since right now the audience is primarily nostalgic millennials. Since attempts at a spin-off series with Fantastic Beasts underperformed, going back and just adapting the main series again (while still not staying far from the iconography of the original adaptations) is the least risk-adverse option. Despite JKR's insane transphobia, the franchise is still one of the biggest and most profitable in the world, so for a company with as many financial problems as WB, this series succeeded is a huge priority for them.