why can't MacBook Pro 2015 use sidecar?

Sidecar uses the Intel Integrated graphics from more modern MacBook Pro to draw the screen for the iPad-as-display.

Required graphics features are not available in older Macs, which feature an older Intel Integrated graphics processor that can not support this trick.

lukearrington wrote:

why can't MacBook Pro 2015 use sidecar? It isn't a hardware issue and it doesn't make sense to me why Apple would limit sidecar to newer models only...

This is limited by hardware.

Use your iPad as a second display for your Mac with Sidecar ...