The usual answer - $. Booster recovery was only marginally worth it for STS - once you witnessed how every screw and nut was taken apart, cleaned, and reassembled, you began to wonder why not just build a new one. That said, it is a wonderful thing for problem resolution if you get the hardware back to look at.
Here are some pictures I took at the STS-124 TCDT showing a recently
recovered aft skirt versus an aft skirt in the process of being refurbed.
With the low SLS flight rate and the economics of reuse bad anyway, it doesn't make sense. But don't take my word for it:
"We don't need to recover the boosters because we have saved enough
booster hardware to advance them for SLS specifications, resulting in
cost savings," said Bruce Tiller, deputy manager of the SLS Boosters
Office at Marshall. "Removing all the parachutes and their associated
infrastructure lifts many thousands of pounds off the vehicle and
improves performance to carry more payload."
Edit: per request, here's a locater diagram showing where the aft skirt is on the SRB.