Before the NCSoft buyout they didn't have the money or staffing. Since then they've upgraded the servers, rolled COH and COV together, hired a TON of new people and have been working not only on a lot of system upgrades but also a whole new expansion (which is likely why the past few issues have been so content light). NCSoft has really re-invested in the game.

Before then? When COH was owned by NCSoft and Cryptic? Nope, they didn't have the resources to do it without sacrificing severely elsewhere.

Worth noting I've got a LOT of COH playing friends who would have to stop playing COH if they did an EVE style revamp of the graphics engine. A lot of my gaming friends in general cannot run CO because the requirements are too high. With bills and family to consider, they don't have the disposable money to buy new computers. Having a less demanding engine has its advantages! Playing COH and CO at the same resolutions on my machine? COH looks a LOT better (details jacked way up) and gets more than double the framerate (CO is usually 15-20 fps on my machine).