Two reasons. First, Summertime Saga is, primarily, drawn by one person. Every single piece of work needed to be done for each update is tied to the artwork of that one person. As a result the work is slower since it's more time intensive than 3D modeling is even if you build the models entirely from scratch. Summertime Saga is the type of project that will quite simply take more time than others.
Second, the larger a Patreon, the less motivation to do things quickly. It's not necessarily laziness, but would you rush to get things done ASAP if you had a bunch of people giving you $614,316 a year before Patreon's cut and taxes? No, no you wouldn't. As a result DC isn't motivated to get things out at a standard clip. Smaller Patreons are motivated to get projects out at a faster rate since they need to give people a reason to subscribe. Virtually every game that takes money from Patreon and other similar services does this. Updates come quickly early on and the bigger and more successful they get the slower and less content rich those updates become.