Because the vast majority of hobby tanks are in that 12-24" height range, and that planted tank keepers tend to have deeper substrates (higher bottoms, less actual distance to plants) than any other application, that it may be only when you get near 30" that you need to think about Halides or more total wattage. otherwise fixture manufactures would probably be marketing their fixtures to address the needs of various heights of say, 12-18", 18-24", 24-30", etc. instead of making it seem like one wattage total fixture is suitable for all heights. for example, the same 48" fixture length can be 2x65 or 4x65 or 4x65+MH, so why don't the fixture makers market them by saying; 2x65 is for 12-18" deep tanks, while 4x65 is for 18-24" tanks, and 4x65+MH is for 24-30" tanks. probably because it's not that much of an issue, otherwise they'd try to make money off most purchasers ignorance. yeah, I know I'm being cynical, but that's the reality of the marketing world.