Because they wanted to have a platform in place first to push the new content towards? Getting software properly vetted and ready to go on the public Steam storefront takes time and effort, and you obviously want all your ducks in a row before you start regularly pushing significant new content to it. There's also the fact that Revival had the updated demo ready to go, and so they'd want to have a place for people who are enthusiastic about the demo to go and purchase what will be the eventual finished product. They'd be losing out on customers otherwise.
Whatever your opinions on early access are in general, it makes a lot of sense for a project like Overload. One of the issues the devs have mentioned about limiting test builds to Kickstarter backers is that the vast majority aren't actively playing them (guilty as charged), so they're not getting much in the way of usable feedback. Opening the test builds up to more public consumption allows for a much larger playerbase. It's also nice in that they don't have to rely on putting out a bunch of disparate test builds anymore (seriously, I have 3 or 4 Overload versions in my Steam library at this point) and can push everything to the early access version. (Apparently the main focus of the initial updates will be the game's endless survival mode, which was just a little bonus thing from the initial preview but got a ton of good reaction. I'm assuming they're starting with that because it enables testing on pretty much all the gameplay fundamentals and balance in a relatively compact setting.) So you get a bunch of new testers to aid the rest of development, plus you get some additional funding while enabling customers to pre-purchase the game and get an early feel for it.
And I get the impression that you don't have much experience with crowdfunding, or at the very least have unreasonably high expectations for it. Gaming or otherwise, I've backed over a dozen projects, the majority of which have already delivered the product or are set to do so at some point this year, and not one of them hit their original projected release date. Hell, I've never so much as heard of a gaming-related project that hit its initial date. Delays happen, new obstacles appear, devs come up with new content that they didn't originally plan on adding. It's pretty much SOP for this sort of thing. Nothing at all suggests that Revival wouldn't be able to complete the game even without the additional early access funding, but a little extra income never hurt, and more importantly it gets them a much wider base of testers. This isn't some sort of SC bull**** where a company is selling JPEGs in lieu of actually developing what was promised. I simply don't see how you have any sort of realistic justification to seek out a refund here.