Probably workload. The fact that he'd be traveling the country during audition period and attending rehearsals/"mentoring" during the August-December broadcast season - I doubt he'll have much time/effort in the radio show and Radio 1 probably wouldn't take too kindly to that seeing as they've given him their biggest show.
Ben Cooper (Radio 1 controller/boss thing) has been desperate to axe older listeners over the age of 30, he's probably persuaded Nick to do the show, and will give him days off when doing auditions, judges houses etc, and perhaps during the live shows give him one week day off (Mon or Fri, which will hopefully mean Scott Mills does the show once a week ) and to top it all off, Grimsaw will get a pay raise because of it. Cooper will instruct Nick to ensure his personality is very teenage girly that it scares people over 20.
In other words, Nick Grimshaw will remain at R1 Breakfast for infinity because of this
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Ben Cooper (Radio 1 controller/boss thing) has been desperate to axe older listeners over the age of 30, he's probably persuaded Nick to do the show, and will give him days off when doing auditions, judges houses etc, and perhaps during the live shows give him one week day off (Mon or Fri, which will hopefully mean Scott Mills does the show once a week
In other words, Nick Grimshaw will remain at R1 Breakfast for infinity because of this