Not sure I agree. Yes the area is flat, but why do we assume that the makers wanted to pin it up prettily on a hillside for all to see? That's just us and our perspective - pictures go on walls and landcape figures like Nazca lines have to be seen from above (you can't get much flatter than Nazca and unless you believe in aliens...)
Supposing - just supposing the importance of cutting a horse in the turf was more important than the finished work of art. Doing rather than being. Plenty of other cases have been suggested where the making of a feature is really more important than the finished object.
Supposing you lived or moved to a hill less region yet your tribal religion, customs, identity badge or totem compelled you to cut a horse in the turf. Now you have two choices -
1 Don't do it 'cos you haven't got any hills OR 2 Do it anyway - on the flat.
I'll not pre-judge but look forward to the results of investigation.