Probably because this:

GregTech ore blocks do not work exactly like other ore blocks. Most mods treat their ores as single
blocks, with nothing special to them except for the few that must be silktouched or can be fortune
mined. All GregTech are non-ticking MetaTileEntities, defined not by block ID or metadata but by
TileEntity data. Each of these TEs contains only two variables: the internal ID of the ore, and a
True/False boolean denoting whether the ore block was placed by world generation or by other
means. This is mainly used to determine whether or not a Small Ore's drops can be enhanced by a
Fortune pick. Their being TEs though has some unfortunate consequences: not all mods are
prepared to deal with TE ore blocks, and so GregTech ores are not always mined by other mods'
automated mining systems.

from http://ftb.gamepedia.com/GregTech_5/Mining_and_Processing

Also does the same with Draconium ore.

Next version of rftools will allow you to enable the ability to mine tile entities. It is still disabled by default due to the possible risks