Probably because the sample time value in your constant block is the default value.
The constant block sample time defaults to inf. The default value of inf indicates that the block output can never change. This setting speeds simulation and generated code by avoiding the need to recompute the block output.
If you want to get the output of the Constant block at each sample time, set the sample time of the Constant block explicitly to the sample time.
In the previous Scope block, the sample time of the Scope block was set to 1 second, so it continuously observed 1.