Because you told it to.
If you run Task Manager, highlight a critical system process like Winlogon, click End Task, and confirm, then gosh darn it, you just killed Winlogon, and the system will reboot. (Assuming, of course, that you have sufficient privileges to terminate Winlogon in the first place.)
Task Manager in earlier versions of Windows would try to stop you from killing these critical system processes, but its algorithm for deciding which processes were critical wasn't very smart, and if you were sufficiently devious, you could fake it out and make your program seemingly unkillable.¹
To avoid being faked out, Task Manager simply stopped trying. Because if you don't do anything, then you can't do it wrong.
¹Of course, it was no such thing. You could still kill the program; you just had to use a program other than Task Manager to do it. (For example, you might try