Simply because more and more triathletes are racing on them. Now that clincher tire quality and wheel options have caught up to their sew-up cousins, the reasons to opt for tubulars have dwindled. In addition to the logistical benefit of riding clinchers (easier tire changes, cheaper replacements), rolling resistance and aerodynamic tests have shown they are not slower than tubulars, although they are heavier (and maybe a little less fun to ride). And if you want to enjoy your expensive set of race wheels more than a few times a year, any of these wheelsets is more than capable of training-day duty.
Why are we still pushing air instead of using computers?
Why, in the era of the iPhone, do bike companies still test and refine their designs in the wind tunnel instead of on a computer screen? Because calculating airflow is insanely complex. Engineers do use computational fluid dynamics (CFD) software to simulate wind drag, but one of these programs would have to track the effect of every individual air molecule on every other molecule to create perfectly accurate simulations. The computing power needed to make that kind of a calculation simply doesn’t exist. So instead, most CFD programs estimate drag by breaking the subject into regions instead of looking at every individual molecule. Even these semi-rough calculations require incredible computing power, and processing each condition can often take hours. These difficulties are why bike and aerospace companies still rely heavily on old-fashioned wind tunnels, even if they incorporate CFD into the process.