6061 alloy is your standard aluminum alloy you see everywhere. 7000 series was originally created for aerospace use. 7001-T6 has about 2.25x the yield strength of 6061-T6.

It is likely that 6000 series aluminum alloy is easier to draw down, meaning that if you make the tubing larger diameter to compensate for it's lower yield strength and draw the tubing walls thinner, you'll have a lighter bike - it would explain why a lot of CAAD frames I've seen coming out of crashes are dented. As for the welds, they just take a grinder to the welds after they are made to make them look smooth. Nothing wrong with that, saves some weight probably, but it is just cosmetic. A welded joint is only as strong as it's smallest cross-section, so if you just grind away excess material, you lose nothing.

The rest is marketing. Fact is that 6061 is the most common Al alloy out there, is cheap, easy to work, but is definitely not the strongest Al alloy around. To translate from marketing speak to real people speak, 6061 is technically easier to draw, so for a $600 frame, they can draw very large diameter tubes thinner for cheaper. It is technically easier to weld, so they can weld the tubes faster, using less weld material, with a higher quality control than 7000 series Al. It's probably a reasonable set of tradeoffs. However, I believe that if C'dale were still primarily in the Al frame business (and hadn't been bought by Pacific), they would have found a way to switch to 7000 series Al. It is maybe harder to draw down and weld, but it is a stronger material for about the same density (meaning higher strength to weight ratio), meaning that if you could find a way to do it, you'd end up with a lighter bike.