Because when you buy it from those other retailers, and you have an issue, you are screwed. 3DR wont even talk to you. I know someone that bought the same backpack/gimbal bundle as I did from 6th Ave, and had a defective battery right out of the box, and 3 DR didn't do anything to help. Entire bundle had to be returned and purchase another (Price increased to $424.99) plus got charged 10% re-stocking fee, essentially costing him an extra $105. In hindsight, he would have been better off just buying two new batteries, and for approximately the same price, would have had a new kit with 2 batteries. If that was a defective unit from Best Buy, NEW or REFURB, 3DR would have sent another battery, no questions asked, and it would have all been avoided.
BTW, show me a "NEW" Solo for $150 from a reputable seller, and I'd probably buy it today.