this math.random script only runs once, even with a loop. why does it do this?

When using while true do loops. You need something to break out from the loop, otherwise it will continue looping forever. So with that in mind, I've gone over your code and implemented a timer for your while loops, so they can break out after a time.

The modified code is as follows:

local textLabel = game.StarterGui.ScreenGui.TextLabel
local eventNumber = 0--math.random(1,5)
local AcidRainTime = 5
local LAZORTime = 2

while true do
    eventNumber = math.random(1,5)
    print(eventNumber)
    if eventNumber == 1 then
        textLabel.Text = "wait whats that falling from the sky"
        local brick = Instance.new("Part")
        brick.Size = Vector3.new(20, 20, 20)
        brick.Position = Vector3.new(0, 800, 0)
        brick.BrickColor = BrickColor.new("Really red")
        brick.Anchored = false
        brick.Parent = game.Workspace

        local touch_event = brick.Touched:Connect(function(hit)
            local hitPlayer = game.Players:GetPlayerFromCharacter(hit.Parent)
            if hitPlayer then
                local explosion = Instance.new("Explosion")
                explosion.Parent = game.Workspace
                explosion.Position = hitPlayer.Character.Head.Position
            end
        end)

        wait(8)

        touch_event:Disconnect()
        brick:Destroy()
        textLabel.Text = ""

    elseif eventNumber == 2 then
        print("Event 2")
        textLabel.Text = "oh no its raining acid"
        wait(3)
        textLabel.Text = ""
        local Players = game:GetService("Players")
        local Workspace = game:GetService("Workspace")
        local brickColor = BrickColor.new("Lime green")
        --// Create a timer since we're using a while loop so we can count upto 5 
        --// seconds before breaking out from the loop
        local StartTime = tick()
        --// create a container for our Rain so its easyer to clean up as the original 
        --// code would destroy the baseplace ... lolz
        local RainBits = (function() 
            local Folder = workspace:FindFirstChild("Rain") 
            if(Folder == nil) then
                Folder = Instance.new("Folder",workspace)
                Folder.Name = "Rain" 
            end
            return Folder 
        end)()
        while true do
            --// grab now time fom tick()
            local NowTime = tick()
            --// subract nowtime from the start time to get the seconds passed
            local DTime = NowTime - StartTime

            local part = Instance.new("Part", RainBits)
            part.Size = Vector3.new(1,1,1)
            part.CFrame = CFrame.new(math.random(-80,80),200,math.random(-80,80))
            part.Anchored = false
            part.CanCollide = false
            part.BrickColor = brickColor
            part.Touched:Connect(function(hit)
                if hit.Parent:FindFirstChild("Humanoid") then
                    hit.Parent.Humanoid:TakeDamage(5)
                end
            end)
            --// if dtime greater than AcidRainTime then break out of this loop! 
            if(DTime >= AcidRainTime) then
                break
            end
            wait(0.05)
        end
        wait(5)
        --[[
            old code use to remove the Baseplate.... FUN!
            for i,v in pairs(Workspace:GetChildren()) do
            if v:IsA("Part") then
                v:Destroy()
            end
        end]]
        if(RainBits ~= nil) then
            RainBits:Destroy()
        end

        print("Event has occurred!")
    elseif eventNumber == 3 then
        textLabel.Text = "someone will get targeted by an orbital strike"
        wait(3)
        textLabel.Text = ""
        local Players = game:GetService("Players")
        local Workspace = game:GetService("Workspace")
        local brickColor = BrickColor.new("Really red")
        --// same idea as the above rain code aka create a start timer
        local StartTime = tick()
        while true do
            local player = Players:GetPlayers()[math.random(1, #Players:GetPlayers())]
            local part = Instance.new("Part", Workspace)
            --// now tick
            local NowTime = tick()
            --// subtract now tick from start tick to get seconds
            local DTime = NowTime - StartTime

            part.Size = Vector3.new(30,1,30)
            part.CFrame = player.Character.Head.CFrame * CFrame.new(0,100,0)
            part.CanCollide = false
            part.Anchored = true
            part.BrickColor = brickColor
            part.Material = Enum.Material.Neon
            part.Touched:Connect(function(hit)
                if hit.Parent:FindFirstChild("Humanoid") then
                    local explosion = Instance.new("Explosion", Workspace)
                    explosion.Position = hit.Parent.HumanoidRootPart.Position
                    explosion.BlastPressure = 200
                    explosion.BlastRadius = 100
                end
            end)
            part.Size = part.Size + Vector3.new(0,300,0)
            --wait(2)
            --// if DTime greator or equal to LAZORTime then remove the LAZOR 
            --//part and break out fom the loop
            if(DTime >= LAZORTime) then
                if(part ~= nil) then
                    part:Destroy()
                    break
                end
            end

            wait(5)
        end

    elseif eventNumber == 4 then
        textLabel.Text = "some dude will get a sword"
        wait(3)
        textLabel.Text = ""

        local Players = game:GetService("Players")
        local randomPlayer = Players:GetPlayers()[math.random(1, #Players:GetPlayers())]
        local gear = game.ReplicatedStorage.swords.ClassicSword
        gear.Parent = randomPlayer.Backpack
        local textLabel = game.StarterGui.ScreenGui.TextLabel

    else
        print("Event 5")
    end
    wait(5)
end

There are still a few problems with the orbital strike Lazar part as it doesn't disappear when the event is over, and you may need to sort out your GUI as this works in the studio, but I doubt it will work online though.

Anyway i hope this helps you and you should be able to read up on the GUI problem through RemoteEvents https://create.roblox.com/docs/reference/engine/classes/RemoteEvent