Data doesn't save upon rejoining, anyone knows why?

local dataStoreService = game:GetService("DataStoreService")
local killSave = dataStoreService:GetDataStore("PlayerKills")

game.Players.PlayerAdded:Connect(function(player)
	
	local totalKills
	local success, err = pcall(function()
		totalKills = killSave:GetASync("Player_"..player.UserId)
	end)
	
	local leaderstats = Instance.new("Folder")
	leaderstats.Name = "leaderstats"
	leaderstats.Parent = player
	
	local kills = Instance.new("IntValue")
	kills.Name = "Kills"
	if success then
		kills.Value = totalKills
	else
		kills.Value = 0
	end
	kills.Parent = leaderstats
	
	local rank = Instance.new("StringValue")
	rank.Name = "Rank"
	rank.Value = "Human"
	rank.Parent = leaderstats
	
	local function updateKills(newKills)
		if newKills then
			if newKills >= 25 then
				rank.Value = "Supreme"
			end
			if newKills >= 100 then
				rank.Value = "Sage"
			end
		end
	end
	
	updateKills()
	kills.Changed:Connect(updateKills)
	
end)

local function save(player)
	local success, err = pcall(function()
		killSave:SetAsync("Player_"..player.UserId, player.leaderstats.Kills.Value)
	end)
	if success then
		print("saved successfully")
	else
		print("failed to save")
	end
end

local function autosave()
	while wait(15) do
		for i, player in pairs(game:GetService("Players"):GetPlayers()) do
			print("saving")
			save(player)
		end
	end
end

spawn(autosave)

game.Players.PlayerRemoving:Connect(function(player)
	save(player)
end)

I have API Services allowed so I have no clue what causes the data to not store.

The PlayerRemoving event is, in some cases, not quite enough to save all data, as the server closing itself will kill this script when it tries to save the data. Sure, you are hardly saving much with that save() function, but saving to datastores can take a really long time as it takes a long time to communicate with the servers. I resolved this problem myself by using the methods in this tutorial, where it is explained that you should use :BindToClose() in addition to PlayerRemoving. Hopefully this helps!

Edit: here is exactly how I do this in my game.

game.Players.PlayerRemoving:Connect(function()
	SaveData(Player)
end)

game:BindToClose(function()
	SaveData(Player)
end)