It's a good question and I'm not entirely sure why that is the case. I think that scuba is such a niche sport (unlike cycling or skiing) that is a very harsh business for a dive shop. Most shops offer instruction as a loss leader to get you to walk in through the door, then rely on selling you MSRP items to eke out a profit. Rent, insurance, staff, etc, add a lot of overhead that Amazon doesn't have to compete with.
That being said, may I recommend you this other light instead? There's a video on Dr. Ben's channel where he raves about it.
Edit: Found the video https://youtu.be/YKFUD7cguSs