This is the best explanation I know:
http://www.loper-os.org/?p=568
In brief: in the early 1980s, home computers were designed with the primary purpose of owners programming the machines themselves. They came with BASIC interpreters and how-to-program manuals. (Examples: ZX Spectrum, Oric-1, BBC Micro.)
But in fact, what happened was that most owners just played 3rd party videogames on them, which they bought ready-made on pre-recorded media.
So late-1980s home computers mostly had much better graphics and sound for better games, and some didn't have a BASIC at all, or only limited ones (examples: Amiga, ST) and better BASICs were left to the 3rd-party market (e.g. STOS, AMOS, GFA BASIC, BlitzBASIC.)
The Mac was on the crux between these generations, with one foot on both sides. Fairly poor graphics and sound, but it did have a (limited) BASIC. It focused on delivering a radically better UI, and this briefly included a radically better end-user programming environment, HyperCard.
But that isn't where the market went, and it wasn't where Steve Jobs was so focussed, which was on the UI and improving it, not user programmability.
Cynical interpretation: making it easier for owners to write their own polished, professional-looking graphical applications would potentially reduce the lucrative aftermarket for additional applications software, so Apple killed off this line of evolution.