My guesses would be:
1. Market maturity and networking effects. Spey tackle is still in a rapid evolutionary phase in the US, whereas the single-hand market is pretty mature. If all your friends bought 9' rods, likely you will too.
2. Difference in market segment preferences. Single-hand rod sales are dominated by trout fisherman. What percentage of those 9' rods sold are 5-wts? Two-handed rod sales I'd guess are still dominated (in the US) by steelheaders, who, on average, are more experienced, more sophisticated about tackle, more prone to muck with their tackle, and therefore more susceptible to be sold on fine technical distinctions.
3. Sample stratification, similar to the previous. The same guys who are buying 9'6" rods for swinging or nymphing and 8' 3" rods for trout-dry-fly are more likely to branch out to two-handed casting. If you look at it from the view that there are two sub-populations, one consisting of the neophytes, dabblers, and two-month-a-year fishermen, the other, the addicts, the difference may not be as large.
4. Physics. Fine distinctions in rod action are more noticeable when speycasting. And two-handed casters are usually working with a fixed-length head, not shooting line in the false cast, not hauling, so there are fewer levers (pun intended) to impact performance.