NintendoPie on 02 June 2013
The reason SSBM improved in the sales department over the original SSB was because the series was new and many people, for obvious reasons, didn't know if they would like the series, wanted to buy the series, etc. So, when SSBM came out people knew they liked it, and they bought it.
The reason for the increase from SSBM to SSBB is that of the Wii's humongous install base. Also due to the fact of newly-implemented Online modes that have attracted a larger (even more serious) crowd, the ones who do it for competition.
I doubt there will be an increase on the Wii U. There will either be a drop to somewhere as low as SSBM, or in between SSBM and the original SSB. (If we combine both the 3DS and Wii U versions it should get upwards of SSBB.)
