"Mathematics" is a singular noun so shortening it to "maths" just doesn't make sense.
For years I've heard the argument that Mathematics should be Maths (being Australian). That's the first time any one has made a sensible argument (to me) for the alternative.
I've never heard anyone call Gymnastics "Gyms". :P
Interestingly, in Spanish and French the unambiguously plural terms 'las matemáticas' and 'les mathématiques' are commonly used, so the 's' in 'mathematics' may be of a different kind than that in 'gymnastics' or 'economics'.