Borthwick, 26, has established himself as a heavy-scoring number three batsman, but his bowling impact has been limited at the Riverside.
The move to a more spin-friendly Oval track could improve his opportunities.
"Playing at the Oval will suit my game as a leg-spinner/batter," Borthwick told BBC Newcastle.
"It's just for purely cricketing reasons, it came down to a massive decision.
"I'm a Sunderland lad, a north-east lad and played here since I was 10 or 11. That's why it's taken as long as it did."
With economic circumstances forcing Durham to be prudent in the recruitment of players, the county's academy set-up has been crucial to keeping Durham competitive.
As such locally and regionally-born players such as Borthwick, Graham Onions, Mark Wood, Ben Stokes, Paul Coughlin and more recently Jack Burnham and Graham Clark have all come through to the first-team picture.
"Durham have been fantastic to me since I was a kid, back from John Windows and Geoff Cook, I'd like to thank them," Borthwick added.
"Then there's Jon Lewis, Alan Walker and Nigel Kent, and Paul Collingwood who have been a massive help to me in my career."
Surrey-bound all-rounder Scott Borthwick says his decision to leave home county Durham this winter is purely for "cricketing reasons".