Most likely because during inspiration you ‘actively’ use your inspiratory muscles to breathe in, primarily the diaphragm and external intercostals. Expiration is generally ‘passive’, because exhaling is brought about when your inspiratory muscles relax and air passively leaves your lungs.

However during heavy ‘exercise’ your expiratory muscles will play an active role in breathing. During forced expiration, when there’s a need to empty more air from the lungs than normal, the accessory muscles of your abdomen and the internal intercostals, contract, forcing the diaphragm upwards and pushing more air out.

Expiratory muscle training is not as widely studied as inspiratory muscle training, but the expiratory breathing muscles are likely to respond to the overload principle of resistance training, in the same way as inspiratory muscles and skeletal muscles.