Back in 2006, when Daz first acquired Carrara, they didn't just get the software. They acquired the entire Eovia company, including some former Metacreations employees who were programmers for Carrara and Hexagon. They had a contract with Daz to continue development of the software, which also included the Ray Dream physics engine and renderer. It's not documented anywhere for sure, but I suspect that somewhere between 2010 (the release of Carrara 8) and today, they lost those programmers or their contracts expired and they left to go somewhere else for more money. That's probably why Carrara has not been updated in any significant way for almost 10 years!

I suspect the main reason Daz wanted Eovia to begin with was simply to have a way to render their products quickly. Bryce was purchased two years earlier for that very reason, but it was just too slow to please most users and there was no way to significantly improve the Bryce render engine without re-writing the entire code from the ground up. Bryce is basically a render engine with features built up around it. Carrara was a modeling package with a render engine built into it. There's a big difference in how you can expand on things there.

Personally, I think Daz should have integrated iRay into Carrara instead of DS as a secondary render engine option. Kind of like how 3dsmax and some other big professional software can offer a number of plugin render engines with their apps. Instead, they must have lost their Eovia team along the way and finding a way to program for Carrara without them proved too expensive or difficult.