Source : http://wh40k.lexicanum.com/wiki/Imperia ... d_Cruisers
Grand Cruisers are significantly smaller than battleships, yet distinctly larger than regular cruisers. These vessels are usually very old in design and do not incorporate many of the features that are typical in current Imperial Navy vessels, like the armoured prow, and are not entirely compatible with current navy tactical doctrine. Due to this, many are retired from active duty, but are still used by reserve fleets. There are also some modernised versions of grand cruisers in service, but since these are much larger and more heavily-armed than their predecessors, they are more often classed as battleships. These kinds of vessels are usually purpose-built or modified from battlecruiser hulls and are not commonly encountered in the Imperial Navy.
Grand Cruisers are no longer used as noted by the various reasons above. The ship has been out classed/replaced by Battleships. Why go back in time?