The first problem would be the height of the intervening terrain and even if you could overcome that, you still have to contend with friction inside the pipe which is a factor most people don’t think about for short distances but when you start trying to carry water long distances through a pipe, friction becomes massive. An ideal siphon inside an ideal pipe is simply a question of height between source and destination. However in the real world, a siphon isn’t unlimited or ideal. There is a height it can’t overcome and it’s actually not very high at all, geographically speaking. The maximum height of a siphon is only around 10 meters. The terrain between the Red Sea and the Dead Sea is pretty flat, but it’s probably not that flat. I’m not going to pretend I’ve done a precise survey of potential routes, but I’d expect there’s probably some bumps in elevation along the way that’s realistically going to need say, 100 meters of lift to overcome. But even 11 meters would simply end the conversation. There’s simply no way around that for a siphon.

The reason for this height limitation has to do with the atmospheric pressure required to keep the water liquid, because once it no longer has enough pressure on it to keep it liquid, it simply vaporizes before it reaches the height it needs to and the siphon is broken before it even starts. In a vacuum, at standard temperature, water instantly vaporizes. The external atmospheric pressure (which is acting on the entire water column up to its highest point, to get it over the hill) is all that is keeping the water in its liquid form inside the siphon. The higher you go, the more work that external pressure is doing, and eventually the weight of the water column exceeds the pressure at the bottom of the water column and again, the siphon breaks.

The friction is the other problem. Even if you could limit your route to no more than 10 meters above the Red Sea, you’re also asking the siphon to not only lift it to that height, but also carry that water through 200 kilometers of pipe or more. We don’t think of pipes as having friction, but they do, and it’s very significant at those distances, especially when your power source (gravity, in this case) is already operating near its absolute limits due to the height problem we already discussed. What you hoped would be a gusher of a siphon will end up being a trickle, if anything at all, with most of the water just sitting idle in the pipe to maintain the siphon while a little dribbles its way slowly through to the destination.

Finally you’ve got all kinds of other more obscure effects at play at those scales, like water’s surface tension, variability of flow rates, possible pinhole leaks in the pipe that will introduce air, offgassing of dissolved gases in the water or even from the pipe itself, and temperature gradients inside the pipe. All of these are going to play havoc with the ability to form and sustain a reliable siphon.

In short, siphons are actually pretty limited, we don’t see much of those limitations on the small scale, but on the larger scale of this project those limitations become very serious, very quickly and basically remove the possibility of using a siphon for any realistic practical water relocation project. Almost all of those go away very quickly when you pressurize the system with a pump instead of relying on atmospheric pressure alone. It’s a fun thought experiment, but in practice a simple electric pump turns out to be a pretty cheap way to solve a lot of otherwise really complex hydrodynamic problems, and when that’s the case, it’s not really worth teasing out a solution to those problems with all kinds of complicated engineering. Just throw a pump at the problem and call it a day, job done.