DHS and DAK are/will no longer be half-day parks. DHS is about to have the most E-tickets of any park in WDW. And DAK is basically already a full-day park.
The existing infrastructure of these parks cannot handle the millions of more guests WDW will need to grow shareholder value. A fifth gate is needed to expand capacity and disperse crowds. Disney's recent upgrades to all four of the parks will make all of them highly desirable, making now the perfect time to build a fifth gate.
Due to Disney's pricing model, it'd also be easy to draw more people away from Universal Orlando. $100 to spend a day at Universal, or $25 to spend an extra day at WDW's new theme park? Of course, Disney would collect far more than $25 after hotel, merch, and F&B. While previously Disney had the limitation of the length of an American family vacation, they no longer have that problem due to the immensity of Orlando crowds.