I need to stipulate at the outset that I am not suggesting, in terms of absolute numbers killed, that members of the Bovidae "family" constitute the most often poached animals on the planet. Clearly they are not. Fish are poached in enormous numbers but we rarely hear about it, or it's not even considered "poaching," per se. The same is true about rodents, amphibians, and birds, as well as the fact that an enormous number of insects (and kindred, tiny animals) are killed (for innumerable reasons) on a daily basis. Likewise, with respect to other large mammals, elephants and rhinoceri are incessantly poached for their tusks and horns, respectively, because of their ever-increasing value on the underground, world market. The same is true of numerous, other (selected) mammals. But, as one single, scientific, "family" of animals, the Bovidae are right at the very top in terms of how many of them are on poachers' to-do lists worldwide. Why might this be?
Well, "family" size is of course one factor. Being a large "family" (143 separate species [Grove and Grubb, 2011, however, using phylogenetic methodology, show that this number is likely more than double the usualy accepted 143\) affords poachers a sizable number of distinct representatives to selct from. However, the poachers don't seem to be all that selective: If they're poaching 46 (at minimum) of the 143 available species of in this "family"--and keep in mind that the total number is arguably higher than 40--then that translates to mean that roughly 30% of all possible bovid species are on the poachers' "hit list" (so to speak); NB: all numbers updated August 30, 2013 to reflect new, ongoing news story inclusions).
On the other hand, there are numerous zoological "families" that are much larger than the Bovidae that do not seem to be a frequent favorite of poachers. Indeed, some insect "families" are significantly larger--e.g., Lampyridae (the fireflies, with a couple thousand species) or Cerambycidae (a type of beetle, with over 20,000 known species)--yet these provide little apparent interest for poachers (one exception being the butterflies--there are six different butterfly families--which are highly coveted by collectors, which means they bring a good deal financial of return on the poaching black market). One might retort, "Okay, but fireflies and beetles aren't anything like bovids; they're not (traditionally) edible, fleshy animals. True, but even if we examine the large mammal "families" (i.e., other mammals, more like bovids), we find far less interest by poachers in such animals as a group.
Some mammal "families" that we might compare to the Bividae are (1) the Pteropodidae (the old world fruit bat/flying fox/megabat family), of which there are roughly 186 known species; or, if one prefers something a bit larger and doesn't fly, there's (2) the Sciuridae (better known as the squirrel/prairie dog/chipmunk family), of which approximately 285 extant species are known; or, for something a bit more "wooly" (more like bovids), (3) there's the Soricidae (the shrew family), containing about 385 distinct species; or, in order to best illustrate how the number of members within a family is irrelevant, (4) one might spy the largest of all mammal "families"--Muridae (the mouse/rat/hamster/vole/gerbil family), which includes nearly 1400 different members. I could go on and on, but the point is that none of these larger mammal "families" are highly prized by most poachers despite their total numbers. (I should mention that I'm not attempting "to compare apples and oranges" here. It's just that there are no other mammal "families" with large animals--ones roughly the size of most bovids--with which one can compare; the overwhelming majority of the larger mammals [unlike the bovids\ belong to relatively small "family" groupings.)
In sum, then, it wouldn't appear as though "family" size is a factor influencing poachers' predilections to go after bovids as a group, rather than target only specific, selected members of that group. So, if it's not "family" size, maybe it's has something to with geographic distribution? I shall explore that shortly.