The thread thus far attempts to put the question into far too simplistic a box.
What must be kept in mind is that the envelope is the critical loci of a series of lines each of which prescribes a limiting (forward/aft CG, and/or min/max gross weight) condition for one design criterion (which is influenced by cg/weight) as required by the design standard. There may be quite a number of such individual lines, none of which necessarily is obvious from a simple inspection of the published TC envelope.
Some of the posts are relevant, while others are somewhat innovatively or mistakenly wide of the mark.
The following is not suggested to be exhaustive, but some of the considerations which feed into establishing the forward limit include
(a) nose assembly components - structural limits
(b) aft body structure for gust loading cases - structural limits
(c) pitch trim capability for various flight stage cases
(d) wing/empennage - structural limits
Bookworm's specific interest as originally posted appears to be the sloping forward cg line at higher gross weights typically seen in GA aeroplanes other than the smallest types. Some of the considerations which may feed into this prescribed limit include
(a) trim/flare capability
(b) nose/wheel/tyre - structural limits
(c) wing/empennage - structural limits
Keep in mind that the aircraft loading configuration, or even the fuel use cg movement, may result in pragmatic truncation of limits - not much point in working up a more extensive envelope than reasonably required. In general, the stall speed is critical for forward cg/higher weight test points - it may be a pragmatic solution to restrict the higher weight forward envelope to avoid undesired performance penalties associated with a higher than desired or necessary stall speed.