Because in Inferno, Life Leech is cut to 20%. I wrote about it under 'sustain' in the initial post. For example to beat 2k LOH, assuming you get a massive 10% life leech :

DPS : 100k
Life Leech : 10%

Total life leeched from damage : 10k life

Inferno life leech rate (20%) : 10k * 20%

Total net life leech : 2k
As you can see, unless you're outputting massive amounts of damage consistently (Like over 100k DPS)...under current game mechanics LOH is more efficient.

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Added on July 9, 2012, 2:13 pm

Damage dealt from Thorns is too low to make it anywhere close to main weapon damage for it to become viable, so there is not much to boost for your 50%. Boost a low number and it will still be low.

Might have been viable in D2 with the Paladin Thorn aura, but not here. If the old D2 Paladin was here, he'd just convert an Elite and slap Thorn aura on it and watch everything suicide on it.

While I don't see it coming anywhere close to being effective in Inferno, I do see the potential in trolling Normal mode.

10k Thorns damage in Normal. Just walk around picking up loot as everything just dies off you. Oh, did we just come up with a potential bot setup?

LoH still better until your barb is dealing dmg in the 6 digits or doing massive aoe.

also, barb's own life leeching skills are based mostly on your maximum hp, and that isn't affected by inferno penalty AFAIK