Designing, creating and testing software, like any other job, can sometimes take longer, even much longer, than expected.
I don't have an inside line to Cakewalk of course, but I wouldn't be surprised if the Sonar4Mac project is taking quite a bit longer than originally expected.
Cakewalk's main product is Sonar for Windows, and it seems Windows updates frequently cause problems. Which means Cakewalk has to develop, test and release patches as quickly as possible and presumably have little or no more warning that a Windows update is going to break something than we do. And the sort of problems that are turning up aren't the kind of thing that could wait a few months before a fix was issued as was often the case on the "old" patch/update system. Which means the developer's plans might get unpredictably overturned at a moment's notice.
That may or may not account for some or all of the delay.
Secondly, there are "alphas" and "alphas". A Mac Sonar that was released as a barely functioning, very buggy and limited version, which much alpha software is, might be a disaster for Cakewalk in the Mac market. If it's free lots of people may download it who are unfamiliar with Sonar and regard a pretty basic early alpha as so bad or under specified they decide Sonar's useless and rubbish then rush off to forums to tell the world all about how bad it is.
I'm guessing that the idea of a free alpha release is in part to draw attention to Sonar and in part to get it installed on as many Macs as possible so any bugs are spotted as rapidly as possible. So Sonar has to work reasonably well on Macs from the day of release while bugs will need fixing quickly if product momentum is going to be maintained.
And the Mac market is awash with DAWs from Apples free Garageband and pretty cheap Logic Pro, which is a powerful, respectable and capable DAW that has a large number of users. Then there's all the other DAWs, all of which except Sonar have been Mac compatible for a while and in some cases for many years. So Cakewalk have to get it right technically and proce Sonar correctly if they want to make headway.
Mac Sonar will happen or it won't. If it doesn't I expect there'll be good reasons why even if they're not shared publicly.