I'm far less familiar with DotA in this regard (and other regards) but I'll hazard a guess.
My not-at-all-certain observations are that 1) DotA's jungle is more complex and perhaps a bit larger and 2) DotA's terrain behavior is more advanced [tree removal, cliffs overlooking the river]. These factors would probably make it more difficult in DotA to place wards in the jungle in a way that is highly likely to yield solid intel — you can get a fairly good amount of safe coverage with a defensive ward on the cliff near river, as well as along the river exit once you've advanced past the river. To the best of my knowledge, you're just not going to learn as much from jungle wards in DotA, because of the sight lines and the difficulty of enemy jungle invasion (and perhaps because of the random invisibility rune spawns, though I don't know how much of a factor that is). Contrast all of that with LoL: accessing the enemy jungle is comparatively easy, and key ward placements on the enemy side (especially tri-brush, blue buff brush, and river junction at wraith camp) are almost certain to be high-information investments.
Additionally, in LoL you have to teleport to a friendly creep, structure, or ward; the secondary nature of a ward as a rally point is something that increases the stakes of ward superiority. In DotA (if I'm not mistaken) the teleport limitation is not the same, so the pressure is not the same. I'm sure there are also itemization and ability factors that I have no knowledge of whatsoever. If I'm horribly wrong on this, I welcome a more experienced perspective!
EDIT: `chc has more specific intel on the wards themselves. The team-wide limit on observer wards is a distinct difference!
> In DotA (if I'm not mistaken) the teleport limitation is not the same, so the pressure is not the same.
If anything, teleports are more limited in DotA. Teleport scrolls can only target allied buildings, or a small area around them. Boots of Travel can also target allied units, but are rather expensive, and thus typically only seen on certain heroes which particularly benefit from them (especially Tinker).
My not-at-all-certain observations are that 1) DotA's jungle is more complex and perhaps a bit larger and 2) DotA's terrain behavior is more advanced [tree removal, cliffs overlooking the river]. These factors would probably make it more difficult in DotA to place wards in the jungle in a way that is highly likely to yield solid intel — you can get a fairly good amount of safe coverage with a defensive ward on the cliff near river, as well as along the river exit once you've advanced past the river. To the best of my knowledge, you're just not going to learn as much from jungle wards in DotA, because of the sight lines and the difficulty of enemy jungle invasion (and perhaps because of the random invisibility rune spawns, though I don't know how much of a factor that is). Contrast all of that with LoL: accessing the enemy jungle is comparatively easy, and key ward placements on the enemy side (especially tri-brush, blue buff brush, and river junction at wraith camp) are almost certain to be high-information investments.
Additionally, in LoL you have to teleport to a friendly creep, structure, or ward; the secondary nature of a ward as a rally point is something that increases the stakes of ward superiority. In DotA (if I'm not mistaken) the teleport limitation is not the same, so the pressure is not the same. I'm sure there are also itemization and ability factors that I have no knowledge of whatsoever. If I'm horribly wrong on this, I welcome a more experienced perspective!
EDIT: `chc has more specific intel on the wards themselves. The team-wide limit on observer wards is a distinct difference!