People are free to cancel the project, there's nothing wrong in that. Maybe situation has changed, maybe they found designer on another platform, there could be million reasons.
As for wasting your time on applications, it's really up to you. If you're desperate for work and will apply to any available offer, no matter if it suits you or not, then sure, you will be wasting lots of time and energy and if you don't have good portfolio, then your chances to win will be questionable, which leads to more desperation. With better portfolio you could apply to less jobs and have more chance to win.
BTW, i agree with you that there is quite big percentage of dubious offers, but with some common sense, it's not that hard to spot them and just ignore. There's absolutely no need to stalk buyers with questions, why they're not choosing you, or why they're cancelling projects.