1. Lack of legs, new Pokemon core titles come out almost every year, while Smash only has one entry per console. A large reason why ORAS sold more than HGSS is because there wasn't Platinum in between them. (Also HGSS were more expensive than the previous games at that point, as it was the first one to be priced at $40 because of the Pokewalker, which hurt them relative to the other games for sale at the time)
2. Increase in price, the jump to $60 from $40, will make it a somewhat harder sell for some consumers
3. Fewer people double dipping, in part because of the increased price and in part because of the extra cost of owning multiple Switches (yes this one is very minor, maybe 100k max, but still it's something)
4. Pokemon Let's Go vs. Smash sales performance, which while not a perfect metric, is the only one we have for the series' performances on the Switch thus far.
5. Smash Ultimate is a far better package than any previous Smash game and all of the third party, and many of the third party, characters that first appeared in 4 significantly broaden its appeal over Brawl, while I don't expect that kind of huge leap in quality/interest for Pokemon. This is speculation at this point though, of course.
I could totally be wrong and Gen 8 might end up blowing Gens 4 & 6 out of the water, there's certainly been enough interest in the series for it to happen, but I expect relatively modest growth in comparison. There are several factors limiting Pokemon's sales growth while all the restraints that held Smash sales back in the past are now gone, although there are things like NSO that could arguably effect both of them.
