Why I can't use let in protocol in Swift?

"A var in a protocol with only get isn't just a let?" No. A let indicates a constant. But that is not the case here. Consider the following:

protocol SomeProtocol {
    var someProperty: String { get }
}

class SomeClass : SomeProtocol {

    var someProperty: String = ""

    func cla () {
        someProperty = "asd"
    }
}

let someInstance = SomeClass()

print(someInstance.someProperty) // outputs ""
someInstance.cla()
print(someInstance.someProperty) // outputs "asd"

The protocol specifies what the conforming class shows to the outside - some property of type String named someProperty which you can at least get.

If the protocol specifies { get } your class can choose to conform via let someProperty: String = "" but it can similarly choose to conform via the above code. If on the other hand the protocol specifies { get set } you cannot use let in the implementation but have to make it set-able as well.

A protocol simply cannot define that a value has to be constant - neither should it, that is an implementation detail that has to be taken care (or decided about) by the class / struct that implements it.

The difference is between

protocol MyProtocol {
    let someProperty: String
}

which makes no sense — a protocol isn't supposed to dictate how someProperty is defined/stored, only that it's available as a property. It could be either a computed or stored property, but that's for the implementer to decide, not the protocol itself.

and

protocol MyProtocol {
    var someProperty: String { get }  // abstract interface
}

struct MyStruct: MyProtocol {
    let someProperty: String  // concrete implementation: stored property
}

struct OtherStruct: MyProtocol {
    let i: Int
    var someProperty: String { return "\(i)" }  // concrete implementation: computed property
}

which is perfectly allowed!

I think a protocol can require that a structure has something, but it can't restrict functionality of struct or object. That shouldn't prevent you from doing what you'd probably like to do in code, for example using a var in the protocol and a let for the implementation is acceptable.

protocol MyProtocol {
    var trythis: Int { get }
}

struct MyStructure: MyProtocol {
    let trythis: Int
}