I don't remember the exact details but CryEngine was built on a variety of assumptions about where tech would go that came to not pass, that's why when they did Crysis for consoles they ported it completely to a later CryEngine and why very few things ever used the first CryEngine comparatively. The CPU thing was they assumed processor speeds would keep going up, rather than becoming more efficient and spreading into more and more cores. It is multithreaded but a lot of the first Crysis tries to draw more effects and stuff rather than become more efficient. To use totally made up numbers if it sees 90% usage it says "okay, draw five hundred more god rays and do another AF pass on the textures, plus tessellate these rocks some more" rather than it staying the same and the framerate going up.
This has happened before with Carmack making assumptions about how hardware would develop. For both Doom 3 and RAGE he made a bunch that turned out to be not how everyone else wound up doing things. The engine for the STALKER games made similar really wrong assumptions along with being busted Slavchunk. (That was part of 4A's defense in the case about whether they had stolen the engine for the Metro games, that it was outdated junk they'd have to rewrite anyway.)
I assume they would remaster the entire trilogy on the newest CryEngine rather than just the first game.
Crysis (and Warhead...and maybe even 2) also has serious DRM issues on certain PCs, not just the engine. That's maybe a better reason to get a remaster than just graphics.
