It did not do that well commercially because there were a lot of people who didn't even know about the existence of Dillon's Rolling Western. And critically isn't that true. At least what I saw in the german game reviews, the games were more the category you love or hate, especially because of the stress situation you're at all the time and also because of the touchscreen controls. Some feel fine with the controls, other ones would have prefered a button alternative control option.
And the first game must have sold more or less good for an eShop title, otherwise they wouldn't have done a sequel as you said. That said, there was already a certain group of gamers who appreciated the first game and bought the 2nd one too. As already said, the 2nd game makes a lot of things better than the 1st one, so if you want to enter the series, you can just jump at the 2nd game immediatly. Also, the games content was amazing for an eShop title. Count the hours you have to play until you can complete one of the both games once. For a game with such a low price, 20-25 hours is quite amazing. Also as you are always in a hurry, you won't be bored by playing the very different stages(especially the stages from the sequel were less flat designed and by that really interesting to roll around)
What is also getting clear when someone reads through this thread: It doesn't even matter if you know Dillon or you don't, his design is so individual and interesting that people get interested in what games the character is from and if Nintendo wants to sell DRW1+2, it would be the most intelligent way to promote him. And even if you don't like DRW, the character is still an interesting one for the roster, also in terms of potential movesets. There is no doubt he would be a great addition, even, if not that many people know him.