For me it is the scale. You can make a scape in a tank like look so much larger than it really is. Plus I can just toss it on a book shelf since it only weighs 20 or so lbs. A ten gallon tank is nice and you can do a lot with it but whenever I see what people do with a 12 gallon long tank, I am jealous. Right now I have a 29 gallon and a 20 gallon long sitting as I try and make up my mind on which one to use for a reef tank. The 29 is the smarter choice due to total water volume but the 20 long is more interesting and will look fuller when fully scaped. Plus lighting is easier (both reef and planted) when you can set the light closer to the substrate.
To each their own, right? I am looking forward to the day when I have a bookshelf full of these smaller tanks each representing a slice of nature. You will find lots of people who love this tank and probably just as many who will poo-poo it because a small volume of water is harder to keep consistent and the lack of fauna that can be put in one, etc. It his a sweet spot for me in between big dreams and little budgets.