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Perhaps there is an obvious reason, and I know that most organisms that are either exclusively sexual or at least capable of sexual reproduction are diploid, but....
Is there a specific reason triploidy seems to be less common than tetraploidy?
It seems like a weird coincidence for a normally diploid organism to produce two rare gametes with extra chromosome(s) at the same time, or to do so at the same time a neighbor does so....
Sexual reproduction means taking chromosomes from two parents. Most commonly, that means that each parent gives just half their chromosomes to their gametes, such that the offspring will have a full complement.
As such, at some point you have to multiply h*2, where h is the number of chromosomes that one parent donates, the "haploid", and 2 is the number of parents. It's possible that 'h' could be some odd number, like if each parent contributes 3 copies in the haploid case, but multiplying by 2 parents always gives you an even number in the offspring.
From there, it's straightforward to either get an extra or lose a chromosome somewhere, but improbable for it to happen for all chromosomes at once.
The Wikipedia page on ploidy is a good place to read about these sorts of things.
