Why doesn't biblatex work with BibTeX?

Up until several (five or six?!) years ago, there was an age when the biblatex package was not yet as mature as it is now and the biber program was, ahem, less than perfectly stable and reliable. Understandably, the default back-end program (to perform sorting) was BibTeX. If one felt a bit adventurous and wanted to use biber, one could (and had to) specify backend=biber to override the default.

At some point, however, biber became the default back-end for biblatex, mainly because (a) it's much more powerful and versatile than BibTeX -- which is, after all, what biber was supposed to be all along -- and (b) biber eventually became sufficiently stable to be usable outside of, to put it delicately, experimental settings. Ever since this switch was made, it has technically speaking no longer been necessary to specify backend=biber if the intent is to use biber as the back-end. Conversely, if one nowadays really means to use BibTeX, one must specify backend=bibtex explicitly. A separate matter: there are by now some biblatex bibliography styles that require biber, i.e., they won't operate properly if BiBTeX is used.

As you probably know, though, old habits -- whether good or bad! -- are slow to die. Thus, many old hands, and quite few younger ones too, still write backend=biber even though it's no longer strictly necessary to do so if the plan is to use the program biber.

As mentioned by Mico, biber is the default choice for backend but one can also give bibtex. As already detailed, that choice was made as Biber offers more features than BibTeX. It is worth noting, however, that biblatex continues to allow the use of BibTeX as a back-end (there are no plans to remove the support). One factor in this is that biblatex was originally written to use BibTeX and that for quite a large number of 'simple' use cases (large parts of natural sciences, for example) the outcomes are equivalent to those for Biber.