Simply putting it, it's a really bad move.
Rototiller raises the Attack and Special Attack of any Grass-type Pokémon on the field by one stage each.
This is the effect of Rototiller, and we can immediately see issues. First it raises both Atk and SpAtk, something that is often pointless as most attackers will focus on Atk or SpAtk, but rarely both. Then it also only raises the stat by one stage, meaning moves like Work Up, Sword Dance or Nasty Plot will more or less always be preferred over Roto. But the biggest issue lies in its limited range. Whilst a lot of Pokemon can learn and use aforementioned moves, Roto only effects Grass Pokemon, meaning it is extremely limited in its usefulness. Even bad stat raising moves like Acupressure effects all Pokemon, unlike Roto.
The ideal Pokemon to use Rototiller on would thus be a mixed Grass sweeper, and there are few to no Pokemon that strategy well.
But now I would like to look at another aspect of the issue. Earlier I said:
...a lot of Pokemon can learn and use aforementioned moves....
I would like to revisit the learn part of that statement. Extremely few Pokemon learn Rototiller, and only 4 of them (2 fully evolved) are Grass types. Those (the Cancnea and Paras family) have other stat boosting options or other moves to spend their moveslots on. And whilst Roto can be used to boost other Grass types in a double or triple battle, it still loses to SD, Nasty Plot or Work Up. And all Pokemon that can learn Roto have better things to do with their limited number of moves. That is, unless they are so bad they shouldn't be used at all.
So what we end up with is a move that don't help anyone, is outclassed by almost everything, few Pokemon learn and no Pokemon should use. Its a forgotten move because its entirely irrelevant, never used and ultimately completely forgettable.