iirc, the reason many hybrids (plants or animals) are sterile is because the parents have different numbers of chromosomes. say there are two parent species, with 14 (7 pairs) and 12 (6 pairs) of chromosomes respectively. their sex cells would contain half the total number of chromosomes (one from each pair), so 7 chromosomes and 6 chromosomes. put those sex cells together, 7+6=13 chromosomes. this offspring would not be able to form viable sex cells, since the odd unpaired cell would make it impossible to divide. (assuming the parents are compatible enough that the offspring even develops in the first place!)
and, in some instances, tetraploids can indeed be paired with diploids to form triploids. these offspring are, again, sterile due to the uneven pairing of chromosomes. however, this sterility can be quite useful. commercial bananas you buy in a supermarket are triploid...hence the lack of big black seeds! same thing with seedless grapes and seedless watermelon.
one amazing thing about the plant kingdom is that pairings such as these are relatively common. there are a few examples in the animal kingdom, but plants generally tolerate it much better.