I suppose it's because Jolly Scarf Lando-T manages to outspeed things like +1 Adamant CharX, +1 Jolly MegaDos (and Scarf base 80s in general), Scarf Excadrill (not the best example, but it used to be a thing), Jolly Mega Lopunny, Mega-Manectric and Timid Mega Sceptile, among others. While Adamant is marginally more powerful (I suppose it's more relevant when RK'ing MegaGross, as Jolly EQ never KOes when at full health), the ability to outspeed all these dangerous threats more than makes up for the loss of power.
EDIT: Greninja'd.
I am surprised that you pointed out the reason of my post,lol.
Basically the reason is to be able kill M-Metagross since it is difficult to one shot him.
The most important ones seem to be +1 Char X, +1 Megados and M-Lopunny.
Char X usually runs Jolly now because of that or double dance set, so running Jolly could be pointless against one.
That leaves with M-Dos and M-Lopunny
Scarf Excadrill cannot kill you consistently anyways at -1 so is not an issue, M-Manectric is not that great anywmore with so many megas,M-Sceptile can be dealt with other things.
I am bringing this up since there are times that you need the extra power of an Adamant nature, particularly against M-Metagross and Clefable who is 2HKOed with SR ups more than half of the times.
252+ Atk Landorus-T Earthquake vs. 252 HP / 252+ Def Clefable: 171-202 (43.4 - 51.2%) -- 62.9% chance to 2HKO after Stealth Rock and Leftovers recovery
EDIT: I also allows you to always 2HKO defensive M-Altaria(taking into account that it has not M-evolved in the first hit obviously) on incoming with Stone Edge, and have 67.6% to 2HKO with Earthquake if it has M-evolved.
252+ Atk Landorus-T Earthquake vs. 252 HP / 136+ Def Altaria: 144-171 (40.6 - 48.3%) -- 67.6% chance to 2HKO after Stealth Rock