Cave paintings such as this are not usually regarded as the easiest targets for thievery, what with the whole "being attached to several million tons of rock" thing. You can't just slip the whole mountain into a bag and quietly walk away. However, the perpetrators had a plan with equals parts finesse and cunning: chiseling it off the wall. In a turn of events that anyone without forethought would call "surprising," this didn't work out, and the lower part of the painting self-destructed into dust, taking with it several millennia's worth of historic significance, the thieves' ability to deny being literally rock-fuck stupid, and civilization's earliest known furry art.
Let's be fair to the numbnuts who did this: Everyone's sort of assuming they planned to sell the painting somewhere, but maybe they didn't. Maybe they were just enraptured by its beauty and didn't have a camera.
"Honey, clear some space on the mantelpiece."
Of course, having a camera doesn't guarantee you won't cause destruction ...