SmackDown! will not be going live on Tuesdays because there's been no reason for it. Part of the reason WWE initially started doing the live Tuesday specials was to test the water and see if the live shows drew better on Tuesdays than taped on Fridays.
With the exception of the first live airing of SD!, none of the other live episodes have outdrawn what the blue brand usually does on Fridays. The last SD! special bombed in the ratings, drawing only about 2.4 million viewers, whereas SD! this past Friday drew right around 3 million. Since this past summer, with the exception of right around Fridays close to the holidays, SmackDown! has consistently drawn 3 to 3.2 million viewers on Friday nights. What that means, in a nutshell, is that it's cheaper to keep SD! taped on Fridays where they consistently have strong viewership. Also, SD! is easily the most watched show on Syfy and has been since it debuted on the network. That gives WWE leverage when times come for new deal negotiations with NBC Universal or if WWE wants to develop a new show or brand to put on another network under the NBC Universal umbrella. Again, just another reason for WWE to keep the blue brand taped and on Fridays for the time being.
When you look at the ratings on Friday nights, the only shows that give SD! competition in terms of viewers are Gold Rush on the Discovery Channel, the popular family friendly line up on the Disney Channel and TBS' airings of The Big Bang Theory. If WWE moved to Tuesdays, they'd have to contend with big drawing shows like Storage Wars and, in the fall, shows like Sons of Anarchy and some others that could bite into their audience.
I agree that Friday night is a weak night on television, comparatively speaking, as more people statistically go out on Friday than any other night. But moving SD! at this time makes no sense. They're drawing more viewers for SD! now than they have been in several years, the show is consistently among the most viewed shows on the day it airs, it outdraws roughly double the viewers of TNA Impact Wrestling, etc. I enjoy SD! a lot most of the time, but the WWE has always generally treated Raw as the "A show" and it's noticable. If they put SD! on equal footing in terms of promotion and general hype, then maybe it could draw more and could result in a move later on down the line.