How come only "Wins" come up on the leaderboard and not all of the ones i created?

I believe it is because you forgot to parent the actual leaderstats IntValue into the player that is joining, try this.

print("Leaderboard loaded")

function onPlayerEntered(newPlayer)

    local stats = Instance.new("IntValue", newPlayer)
    stats.Name = "leaderstats"

    local coins = Instance.new("IntValue", stats)
    coins.Name = "Coins"
    coins.Value = 0

    local diamonds = Instance.new("IntValue", stats)
    diamonds.Name = "Diamonds"
    diamonds.Value = 0

    local wins = Instance.new("IntValue", stats)
    wins.Name = "Wins"
    wins.Value = 0

    local level = Instance.new("IntValue", stats)
    level.Name = "Level"
    level.Value = 0

    local exp = Instance.new("IntValue", stats)
    exp.Name = "Exp"
    exp.Value = 0

print("Leaderboard Fully loaded")
end

I also removed the bits of code you had at the bottom because doing what I did is more efficient.

When you put a , after a instance you are creating you are setting its parent.

Here is an example:

--x = Instance.new(InstanceName, Parent) 
x = Instance.new('Part', game.Lighting) -- This will create a part and instead of it being put in workspace, it will be placed in the lighting. 

This helps eliminate lines of code therefore reducing reading and also removing room for error in your code. You should stick to this method of setting parents for new script generated objects from now on.

Hope this helped!