The LEGO monorail was never a theme in itself, but took form in three main sets released for the 'Classic Town' and 'Space' themes (6990, 6399 and 6991) in the years between 1988-94. A few accessory packs with extra track elements (6921, 6347) were also released in the same period.
Since then, TLG stated for several occasions they do not plan to produce new LEGO monorail sets, despite continued requests from the communities of LEGO fans.
There might be several justifications for this decision from which the most plausible ones that I'm aware, include the high production costs of the monorail track elements, mainly because of their complex and expensive molds which meanwhile ran out, and the low production rates from these molds.
Although if we look at LEGO patents we can find that originally TLG even had plans for additional track element types.
# Actuating device in a toy track assembly #
We might think on several other reasons like competition for market share with the LEGO 'Trains' theme, not enough sales volume foreseen (profitability) and finally the latest kid generations not finding it appealing... even rather boring.
Although there are no immutable facts, definite decisions neither decisions that we can't challenge. LEGO CUUSOO could be a suitable platform to raise that challenge and show TLG there is still huge interest and market to get the monorail back.
Personally I believe a monorail set is something still appealing and worthy to many!
The monorail track system is more advanced than train tracks in any layout. The mechanical automatic functions that allow the monorail to move back and forth, or change lines, is something unpaired with any other LEGO track system.