That SCR has a "typical" holding current of 5mA so you are
marginal for getting the SCR to latch on (. Try reducing your
1K series resistor to (say) 100 ohms. It's not like you're going
to "crowbar" the supply with all that series resistance anyhow,
why is it there in such a high value?
I suspect that the SCR model may be a bit funny, the 5.001
clamped voltage is just a little too cute and too close to
the 1K * 5mA holding current result. Maybe the model
deserves a closer look, re realism.