It's sort of an unwritten rule, like 2500 touches or reaching 30 years old.
The punishment takes it's toll quickly, Here is a link. There is a chart that shows 300+ carry backs and how they did the next year. Teams are reducing the load because that number of carries, especially if the runner is a physical type of back, catches up fast.
The chart didn't paste in well but you can click the link and check it out. Gurley isn't a highly physical runner but he is moderately so. I'd like to see him get more rest so to speak.
http://www.espn.com/blog/philadelph...t-legarrette-blount-and-the-300-carry-concern
The Year After 300
A look at running backs with 300 carries in a season since 2012 and how they fared the season after:
PLAYER SEASON CARRIES YARDS TDS NEXT SEASON YDS NEXT TDS YDS DIFF TD DIFF
Doug Martin 2012 319 1,454 11 456 1 -998 -10
Alfred Morris 2012 335 1,613 13 1,275 7 -338 -6
Arian Foster 2012 351 1,424 15 542 1 -882 -14
Adrian Peterson 2012 348 2,097 12 1,266 10 -831 -2
Marshawn Lynch 2012 315 1,590 11 1,257 12 -333 1
LeSean McCoy 2013 314 1,607 9 1,319 5 -288 -4
Marshawn Lynch 2013 301 1,257 12 1,306 13 49 1
DeMarco Murray 2014 392 1,845 13 702 6 -1,143 -7
LeSean McCoy 2014 312 1,319 5 895 5 -424 0
Adrian Peterson 2015 327 1,485 11 72 0 -1,413 -11
Ezekiel Elliott 2016 322 1,631 15 N/A N/A N/A N/A