- People are more likely to choose a Pokemon that can actually attack then one who need to rely on stalling to KO Pokemon
- For people who actually want to use Bulky Pokemon / Walls there are better choices. Seismic Toss, Night Shade, Leech Seed, ect.
Poison isn't the best Status to inflict. Sleep, Burn, or Paralysis cripples a Pokemon more
Not exactly true. Toxic is a great way to take out any tank/wall, since it builds up damage over time. It's not just regular poison. Remember that each turn adds another counter. First turn, 1/16th HP damage. Second, 1/8th damage. Third, 3/16 damage. Fourth, 1/4 damage. So on, so forth.
As for status effects, Freeze is too unreliable to count on. Burn is only good against physical sweepers. Sleep is often restricted to one Pokemon at a time, because it IS so good. Plus, sleep got nerfed this generation. Paralysis is really best used against faster Pokemon, which generally excludes walls, and the "Full Paralysis" chance is too low to be reliable.
Now, granted that Steel types are immune to Toxic, and generally make great walls (Steel is a great defensive typing), but being weak to Fire, Ground, and Fighting is often an easy way to work around that. Aside from Steel-types, Poison-types are generally easy to KO anyway. And anything with Poison Heal is often going to be better off Paralyzed or Burned.