Why are tennis outfits always miniskirts, I wonder?

Literal

Tennis-wear [topic-って] why miniskirt [copula-な][wonder-のかしら]?

テニスウェア is a wasei-eigo compound — 'tennis wear' (English would say 'tennis outfit' or 'tennis dress'). なんで is the casual equivalent of どうして or なぜ. のかしら is the feminine wondering form. A playful musing about sports fashion conventions.