Simply because they are extremely popular brand names.
The R4 got cloned so badly because it used to be the #1 flashcart, and most people refer a flashcart as a "R4".
But know since we are making n00bs stay away from R4's and go to acekards, clone company's are making acekard clones now (Acekard 3DSi, Amazekard etc.). They are relatively easy to clone but most of them can't run akAIO since the clones are made from other cheaper parts.