This has been asked and answered a thousand times, but I'll say it again. Downtown Atlanta is literally the least convenient place to go in the metro area, mainly due to horrendous traffic. Also, downtown is quickly dying, as it's mostly vacant office towers by now. There's no vibrant scene or neighborhood near the arena and stadium, just a sea of empty parking lots. Google Atlanta's "Gulch," and you'll see what I mean.
As for "trekking," you misunderstand. It was a trek to get downtown. The people mostly likely to go to NHL games would much prefer to have the arena right around the corner from where they actually live. I'm a perfect case in point. For me, if I didn't already work downtown, it's at least 90 minutes to get to SFA from my house during afternoon rush hour traffic. The proposed site for Anson Carter's group is literally 10 minutes from my house, door to door. If I had to go up to The Gathering in Forsyth, it'd be about 20-25 minutes, with traffic. That's a huge difference from having to go downtown.