Built-in obsolescence, which can be in various forms. A new or rebuilding club attracts a lot of builders, guys who like making benchwork, doing wiring, laying track, etc. Then the layout gets finished, and re-doing or changing finished parts is not allowed, so builders get bored and leave. Someone also mentioned old layouts that are wearing out, or have areas needing reworking or modernizing (scenery or electrical) but the old-time members resist any change, so the newcomers with ideas leave (that's what I ran into). Or a layout is built and runs well, but when operations-minded members try to run they find the layout is set up poorly to do so. This is also a matter of what kind of railroaders get to design the layout and how it can be used: roundy-roundy types that design industry spurs (and very few of them) only as scenery they pass with their non-stop trains, or operations people who want to work every piece of layout track. The truth is model club membership interests and abilities change as members come and go, but rarely is the layout changed to reflect that. Modular clubs have an advantage in this regard. Though I've been in modular clubs where some people have very toy-like scenery and marginal trackwork, which clashes badly with somebody's super-detailed and realistic-looking module. Another reason is bad location. When it takes everybody a long drive to get to the club, the price of gas takes a toll on attendence.