Could have been a thematic decision - they wanted to hide the first turntable as much as possible? The 2nd one doesn't matter as it's dark & you can't see it anyway, so they went with a more reliable option
I have been thinking about this and looking back to my days of doing hydro dynamics - apologies if it goes in depth a small bit.
I think the reason they have two vastly different turntables is quite simple, it has to be to help the flow of water. The V turntable would not be affected as much by water flow as due to the shape of the turntable area, the water still has plenty of ways to travel around the boat when a boat is in the turntable. You see this not only on Valhalla but other V shaped turntables, such as Storm Force 10, which has a submerged V table also. If they had the track going in the same configuration as the first turntable as the second, they could have the same turntable setup.
Put yourself into a 180 degree turntable though and it is a very different kettle of fish, if the turntable and boat are fully submerged, as the boat rotates while submerged, it is going to temporarily create a chokepoint for the water, especially as the boat is momentarily side on to the water flow. These flumes have quite a high flow rate, that water almost straight away is going to attempt to go somewhere, it will attempt to go into and around the boat and over the edges of the channel. You cannot counteract this by making the channel wider either, because that extra width will result in a much lower flow rate and depth of water on that section of the flume when there is no boat there as the water is free to flow without congestion. To the point that when a boat arrives there, there possibly wont be enough water to move the boat onto the turntable. I mean yes, you could possibly counteract this with more tire drives, but you then have to also compensate for when the channel gets thinner again and all the water related issues that come with that.
Much easier to raise the boat out of the water and avoid all this completely. It is a common feature on a lot of their flume rides, so it certainly is not theme related.
A very elegant and simple solution from Intamin if you ask me, to what could be quite a complex hydro dynamic and engineering problem