This is seriously stupid easy to answer.
When eve was first written in 1997 (which is when they started making eve) the Mantra of the day was MORE GHZ!!! Multi core processes were no where on the radar at the time. so eve was written with the expectation that processors would keep getting faster and faster base don increasing GHZ. The first multi core processor was released in 2005, 2 years after eve was released.
Second issue, eve built on its self int hat 2 years they released three expansions. One of which added nasty code, known as PoS. By the time the multi core became common, it would of taken way more work to redo the entire engine as well as the base code and all its system then to just keep ploughing ahead and adjust your systems to try and use things better.
Thirdly, a rewrite of that type would of taken a lot of time and resources, and while eve was growing, it was smarter to add more to eve and get more money, then to halt new stuff and upgrade the core code to use mutli cores. Because lack of new stuff would cause people to leave.
Flash forward to eve’s 10 year, and the phase we are in now. We are technically in a rebuild and repair phase, and its taking forever. Every thing they change, most likely is being optimized to use multi core processors, so when they do upgrade the enging and core code, they are ready to go. The issue that has held them back, imo, is the pos code. Its a mess of spaghetti and is tied to all kinds of systems. At one point a few years ago, they tried to make a small change to the pos code, and wound up killing both mining and industry. They also had zero clue why. So they repaired the glitch and left it alone. This is why you saw rewrite of some systems like industry, and sov come before pos changes. They have to get thous systems away from the pos code so they can kill it. I believe, once the pos code is gone (deleted, commented out, or just ignored) we will see new stuff come.
But thats the main reason form what i see, they wanted eve to grow, but at some point the elephant in the room got too big to dodge anymore, and here we are.