I think the simplest answer is because it's a new meta that people are still adjusting to. Lane swap strategies are complicated beasts and constantly evolving in what the "appropriate steps" are to play it out. There were periods of last season where they randomly left the meta, but Korea pulled them back out like SSW at Worlds.
I'll go into a longer answer below.
You lane swap for several reasons:
- Suppress a top laner's power spike
Most comps are trying to scale now. You don't have comps with an Irelia mid game power spike that you want to stall out so long that she misses it, giving your top laner a better chance to outscale. You both want to scale, so you don't gain an advantage over the opponent lane swapping.
- Accelerate the game's pace by causing towers to fall early
You want a faster game when you have an earlier win condition. When you're on a scaling comp, even though you can end the game earlier, it's not a necessity. You can farm it out and let your tank jungler reach level 6. When you're trying to force a mid-game snowball, you want more gold in your pockets to hit your power level 11-16 power spike swinging, and because usually an assassin is involved, you want open maps to apply more pressure.
- Avoid a bad lane match up
This can still occur, but probably only if it's a really punishing match up.
- Because the other team doesn't know how to handle a lane swap
In the West, you could gain significant advantages by lane swapping solely because the other team would drop the ball on it, either misplaying a wave of CS or misreading an incoming dive. I actually think a team like LPL's Vici Gaming could still wreck many teams by lane swapping and playing it better than the opponent. Another component here is adaptation: Not only are teams learning the lane swap, but the champions in the meta now are comfortable jungling (eg. smite top).
- The side of the map may matter.
I don't know enough about the details here, but I believe being on blue side makes the lane swap less favored because you don't access the dragon pit as easily. I'm not sure on this one.
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##Finally, there's the downside of lane swapping:
The team that opts into a lane swap loses dragon control.
This may not seem like a huge deal, but missing first dragon can be a pain in the ass. Not only do you gift over the dragon buff stats, but you lose the timer on it which makes contesting it in the future more difficult. Additionally, if the other team takes dragon quickly enough, you may further jeopardize the 2nd dragon if the 2v1 is somehow still in effect 6 minutes later. In general, the other team will have the timer and full vision control of the bottom half of the map, and in order to ever contest dragon again, you will have to make a coordinated effort to overcome this at some point.
In the double Smite meta, this is a pretty dangerous scenario. Even though 5 buffs seem like a forever-long clock, it actually ramps up pretty quick -- you can be on 3 dragons by 15-20 minutes easily, which is a decisive advantage for the other team because they can start dictating your map movements at that point. You don't want them to hit 4 dragons, so you feel obligated to fight, and if you don't fight, a short 6 minutes later you get destroyed with a 5th dragon or a forced baron trade.
In the previous meta, games were already being decided in teamfights by 20-25 minutes, so as long as you were trading towers for the first dragon you were ok. Also the "dragon race" was being delayed because first dragons were taken later due to a preference for pushing the sidelanes and sending the jungler to threaten the dive, deny farm, and take the turret before rotating to dragon. They did this commonly because late invades used deep wards to guarantee the lane swap, which caused people to arrive to lane late and miss the freeze, forcing you to set up a 3v1 push. As people have adapted to lane swaps, they stopped caring about the deep ward and just defaulted to a 3v1 push if the enemy jungler went bot. At various times, teams have also sent their junglers to reinforce the 1v2 lane to stall the push while the other lane maintained a freeze, which makes freezes more effective. By setting up a freeze, you open the possibility of first dragon being taken very early. Nunu becomes really popular for this. It's a back-and-forth, evolving permutation of the lane swap; probably pretty soon you'll see the lane swap jungler setting wards to contest the sub 5 minute dragon or roam bot to save the turret. Gragas is really good for contesting the enemy jungler early, and we've seen how popular he is. He also functions well in standard lanes because of how he out-pressures the other jungler.
So what about Western teams still running lane swaps? I'm not convinced it's a poor meta read; it still has its uses in the right match ups or against certain players (like Huni or Impact). I showed in the last paragraph how convoluted lane swaps can be, dependent on so many factors, that I think teams could still be taking advantages over opponents with a smartly planned comp and well-executed lane swap.
Who knows if the East is just culturally inclined to go head on (head-on confrontation is how LPL teams gain most of their advantages anyways), or whether their lane swap iterations hit a dead end, maybe an uncounterable step like dragon being taken early without anything for the lane swap team to show for it, so they found the 1v1/2v2 preferable. It's hard to say. There are still Eastern teams who pull it off really well though, and I wouldn't rule it out appearing at MSI from an Eastern team, or at a later date in the year.