Why limit the bank angle to 10° when passing through FL300 on a B737-800? When passing thought FL300, we are instructed to set the bank angle limiter on the MCP to 10°.

I know one of the reasons for this is that we are thrust limited so maneuver capability decreases.

Are there any other reasons?
One reason that comes to mind is as you get closer to the coffin corner, the extra G force due to maneuvering could cause a stall.  Quoting from "Stall the Pig",


  In the thin air at altitude, jets operate within a very narrow
  airspeed band between MMO (redline) and the low-speed limit, typically
  a yellow arc that provides a warning zone prior to a stall, indicated
  by a lower red line. At high altitudes and high weights, this
  band—known as “coffin corner”—can be quite narrow, leaving you a mere
  10 knots of usable airspeed range, for example. Load up the wing in a
  turn and that lower redline will quickly move upward toward your
  airspeed.


By limiting the bank angle, you also limit the G forces, and stay farther away from an accelerated stall.