Best version of Sonar that Cakewalk has ever done. That's why.
It's a no-brainer really, IMO. Snappier, more responsive, more stable, more reliable; all new additions are off-the-charts humongous... more refined and slicker user-interface, better midi input method, superb REmatrix Solo, excellent Addictive drummer 2, Craig Anderton Amp sims... all these are off the charts good, and all for $149. Arte you daft? Just kidden, but seriously, unless you're strapped for cash, can't really go wrong man.