I guess when a title is stupidly long, you reduce it to something easy to reproduce countless times in everyday conversation. Last time
Forum had a major Broadway production, the internet was not used in the way it is today. Back then, not everyone had the internet, and not everything was reduced down to acronyms. So the sensible way of abbreviating it stuck. Whereas
Sunday has had a major production since the internet became ridiculously commonplace, since acronyms became the conversational norm, so it made perfect sense to abbreviate it down to SITPWG.
In answer to the question posed by the thread title, one word titles don't tend to be abbreviated, but two and up is apparently fair game. Although foolish, as LM could mean
Les Miserables (itself a popular pre-internet show with pronouncable abbreviation) or
Little Mermaid (although TLM is a bit more common for that). Ideally, it'd be kept for titles with five words or more, but people are apparently incredibly lazy.
