A greater percentage of STHs means less members which means less money for the club
Indeed. Plus, the spend per head of an average ST holder at the match will be less than a non-STH. A typical STH will probably drink elsewhere, and maybe get into the ground 10 or 15 minutes before kick-off. Someone who comes maybe once or twice a season (maybe even a lifetime), possibly brings their kids will not only get in the ground earlier and buy food/drinks in there, they'll probably spend money in the club shop too before or after the game.
While our smaller capacity holds us back in terms of match day revenue compared to say Man Utd or Arsenal, it means that the club don't need to worry about selling out - the majority of games do. Combine that with the popularity of LFC, and they simply don't need to allocate as many STs as other clubs. Everton, for example, have more STHs than us on a smaller capacity. It's better for them to have that money guaranteed rather than rely on single ticket sales during the season, plus they won't have as many tourists going to the match as we do. On the flip side of it, Man Utd don't need to worry about their popularity but their stadium is huge, and a poor run of form (like at the moment) means that sales could suffer, and they'd have empty seats as a result, so it makes sense for them to have a lot of STs too. We're in a weird sweet spot in terms of capacity and popularity that no other club in the league is.
