At the same time, a lot of the slightly slower threats have just enough bulk to tank a hit if need be and do something back. (plus, Rock Polish is a free turn so there is still a risk to setting up) For example, all the S rank Pokemon (I'm just going to scroll down the Viability list) either outspeed unboosted Lando-I or have the natural bulk to tank an attack.
In A rank, Loom is going to be a coin flip between attacking or boosting, same for Dnite except he has Espeed as well, Ferrothorn is screwed by FBlast (but it can still be trolly with Seeds or Gyro), Heatran without Ballon is going to be taking a heavy risk trying to attack, Spdef Hippo can tank a blow and bite hard with Ice Fang (otherwise this is safe setup), Rachi has Body Slam to watch out for but it's obviously not liking EP, Kyruem-B is dangerous to setup on but Focus Blast nukes it as well as EP after SR (often times at least), Lando-T is only a bother if it has HP Ice, Ninetales is in a tight spot and probably not going to stick around to attack, Mence is almost always scarfed but otherwise 101 trolls it, Spdef Skarm is going to be annoying but Physical is setup fodder, TTar is worried about EP but it can bite if Lando RPs. Everything else in A tier isn't going to be something Lando is going to want to stay in on. (Cept Celebi for 4 attacks) That's only 12/21 Pokemon and that is assuming they don't run some of their more common sets.
B rank has a lot of those defensive checks/counters that are mentioned here. (I guess that does say something about Lando though) There's only Forry, Gliscor, Haxorus, Kingdra, Hydreigon, Kyurem, Lucario, Zone, Sableye, Tentacruel, Toxicroak, and Volcarona that are going to be bothered by 4 attacks. That's again just 12/32 assuming they don't run Scarf or something and just about all of those don't really scream setup fodder either.
Basically, Lando-I isn't setting up a Rock Polish willy-nilly without any risk involved. Just about all of these are slow priority abusers or defensive Pokemon (that have a glaring weakness or don't invest in Spdef) which pretty much confirms that Lando-I has the raw power to dismantle defensive cores but it is going to have a hard time doing much against offensive teams. (without the Rock Polish boost) At least Deo-D didn't care if it died laying down a faster layer of Spikes and it could also pull out random victims with Red Card. Most players are going to be fairly conservative with the Pokemon that Lando-I can setup on (not to mention a lot of these are hard to switch into) which'll starve it of setup opportunities and, thus, playing time. Not trying to say these sets are crap or anything but the 4 attacks is definitely hurting its general viability by throwing out RP and even the Rock Polish set isn't exactly brain-less. (or, well, no more brain-less than most of OU)
Edit: Just a quicky but reason #1 isn't exactly accurate as priority attack will have successfully forced Lando-I out. Again, it's too slow in the Offense match-up so losing that Rock Polish boost is a pretty big deal for it. #2 Either lists suspects banned in BW1 (a very different metagame) or Pokemon that didn't mind a forced switch as they weren't reliant on a boost to pose or threat or hazards weak. (Tornadus and Excadrill for example)