Consider it as cheapening the effect of the word I suppose.
To me at least, genocide is a powerful word that is suppose to draw out the incredible hatred, intensity, size and calculated cruelty of eliminating a whole group of people for having one or more pertaining characteristic, often used with a specific political or social goal in mind.
Rather than just 'massive murders and killings'.
The massive death toll in the Thirty Years War was just due more to indiscriminate killing and murder, sacking of cities, disease and starvation than an actual concentrated attempt at trying to wipe out 'A' group of people. Sure many were killed because they were 'X' religion, but it's not like a Catholic Army conquered a town and then proceeded to construct an entire organisation whose sole purpose is to drag out and kill Protestants as oppose to just looting everything in sight and killing people who they think are Protestant on the spot.
To narrow it down, IMHO, is to exactly pinpoint a hate, an evil of man, so strong and so disgustingly terrible that man can be driven to organise a killing machine, specifically to destroy a specific group of people.
If we just loosely and broadly applied it to 'a group of people who killed another group of people because they were 'X' people' just weakens it.
Soon we will 'genocide' grass when go mow lawns. Like how we now 'crusade' over everything.