Why do you hit MMO at constant IAS when climbing?

[WhySoTough]

Just looking for clarification.
At about 25,000 feet Pilots change from indicated a/s to Mach, reason being that You can hit MMO at constant Indicated Airspeed during climb, and the opposite (VMO at cnstant Mach number) in descend.

Can someone be kind enough to explain to me why this is?

Thank you.

[LVL_CHG]

http://www.pprune.org/professional-pilot-training-includes-ground-studies/487511-four-finger-rule.html