Passive 2-phase immersion cooling is not new, but the simplified way how we use this technology for cost saving is new. So why hasn't anyone done this before?
In short, the computer and chip technology has matured in the past few decades. What we couldn't do just 20 years ago is now possible due to modern chip packages that incorporate heat spreaders with high performance interfaces are routine. Other factors are the increased demand for high density (ie. GPU computing) that drive our developments, and a few very simple things such as the availability of SSD for storage or new types of hard drives (Helium).
Here is an article with more technical details you might find interesting (starting with IBM's research into this area in the 1970s):