^^^ Probably a reasonable assessment. Better/seasoned players tend to prefer 6-max. Post-BF, the ratio of fishy recreational players to serious grinders contracted a lot, so I expect the demand for 6-max games grew while FR demand dwindled. And yes -- with the much smaller player pools, it also takes much longer for the tables to fill.
Funny though, I see the opposite when it comes to DONs and STTs on Revolution. The FR DONs and STTs I play there ($15-$60 BI's) seem to be more popular than the 6-max, which is frustrating as I prefer 6-max.