BASIC was one of the standard tools for 3rd party software. It was pretty easy to learn (compared to C or FORTRAN) and was migratable (I wouldn't say portable, but stuff written in certain dialects would generally be fairly straightforward to port to slightly different dialects or reimplementations on other platforms).
Not to mention a whole generation of people (myself included) for whom BASIC was their first programming language. To this day, I remember my first program:
10 PRINT "STEVE IS THE BEST!"
20 GOTO 10
A lot of computer games in the early 80s were written in BASIC with bits of machine code (no pun intended) thrown in for effects, copy protection etc.
Of course, the pinnacle of BASIC programming came in the form of QBasic, specifically GORILLAS.BAS.