I think the heart of the problem is that the rudimentary ideas of forth- a stack, some stack-shuffling words, some arithmetic, control flow, and that's about it- are appealingly simple, but not enough of the language:

>"I figured I didn't need the more interesting metaprogramming stuff for my first prototype programs, and I could add it later if it turned out that I was wrong. It was wierd to throw away everything I originally liked the most, but I was all set to start writing real programs. Solving real problems in the real world. It was among the most painful programming experiences in my life."

Imagine reading the first chapter of SICP without prior exposure to Lisp, excitedly writing an interpreter for the parts of the language you'd seen so far, and then feeling your enthusiasm slowly slip away into dismay as you attempted to use that stunted fragment of a language to get work done.

Forth can be small, and simple, but the real gems of the language are a bit subtle. The many implications of CREATE>/DOES. Abuses of manipulating the rstack. Striking a balance between parsing words, named locals, global variables, vocabularies, and other tools to minimize twisty stack shuffling. Bending the rules at the right times.