I ran into this same problem today. I tried joining both data sources on the fields I thought necessary for Tableau to figure out that it should give back the 'show fewer values' quick filter option, but it didn't work. I renamed the variables so that they were identical and I even let Tableau do the join automatically, that too didn't work.
When I think about what Tableau has to do to figure out to show fewer values in a filter, it would seem possible given a data blend.
Could this be a bug?