I mean, you could pay to get all the rankings details from USNews, and that would tell you why they ranked Bates a bit higher. But would that really answer your question?
I will say that my own two cents is Bates has long been known as a very academically strong college, which also has a lot of appeal to certain people because of its setting including proximity to lots of outdoor recreation.
I also note it was early on known as a relatively progressive school, including being quick to admit women after it evolved into a full LAC, and I believe never had frats or sororities. It was therefore sort of positioned as the more progressive and middle class alternative to the then more conservative and upper class Bowdoin. I think in some ways it moved more upscale over the decades, but I think it still retains at least some of that branding, which again appeals to some people.
I actually like Connecticut College’s location (New London is a cool small coastal town), and I think it is very good academically too, but maybe with not quite the same academic reputation as Bates. It also has a pretty different history–it was founded as a women’s college when Wesleyan went male only (ending what was known as The Wesleyan Experiment), which is relatively recently as these things go. So it doesn’t have quite the same deep history. I also think it faces more direct competition given the proximity of so many other colleges, including now Wesleyan again since it is back to coed.
Does this historical comparison help? Maybe not either. The fact of the matter is there is huge demand for good LACs in the Northeast, so plenty of room for both Bates and Connecticut College and many more. Some people may favor Bates for various reasons, but I wouldn’t let that concern me if Connecticut College was right for me.