The Smalltalk programming language and environment was revolutionary at the time and still highly influential to this day. Lots of later languages have copied some of its features, and none of them really got it right.
The grammar is extremely simple and easy to pick up compared to most industry languages. A famous small program demonstrating all of the language (but not the library) fits on a postcard.
Using the debugger, you can catch an exception, walk up the stack, and correct, recompile and swap in individual methods while the program is still running. You can save the entire state of the program at any time and resume it at a later time, even on another machine. You need an entire OS in a VM to do this in almost any other language.
The tight feedback loops you get from its interactive programming stye is arguably superior to almost anything else we have today, although e.g. Python or ClojureScript can approach this level of interactivity, it isn't their default.
Smalltalk's first stable release was in 1980 and we still haven't caught up to its level in industry. It's hard to understand exactly how this happened historically, but it seems to be path dependence based on some combination of (relatively) poor marketing, early mistakes in design, and the limitations of older hardware that could barely handle those features when the industry was first taking off.