I cannot believe it didn't even do half of what Alice in Chains did. That just makes no sense to me.
The only way that makes sense to me is if STP has a much younger fanbase than AIC. Which could be the case.
Think about it: While AIC has mostly stayed the same from album to album, STP has changed their musical direction with each album. For example, if someone loved Core, would they also love Tiny Music? I'm sure at least half of the people who loved Core and got into STP soon after Core was released didn't like Tiny Music at all (or #4, or SLDD, or STP, etc.).
I'm sure STP has lost at least as many fans as they have gained with the release of each album. This could mean that STP's fanbase is skewing younger and younger with each successive release. Younger, more internet savvy fans, who are much more likely to download an album than buy it.
I'm sure STP gained a lot of fans because of VR, but probably younger fans. Maybe why Contraband sold so well was because we had the GNR '80s fanbase helping us out.
STP is very popular. I would say more popular than they were when SLDD was released (as evidenced by the chart position of BTL). Unfortunately, this does not translate to album sales.