Power requirements, moving mass and audio sampling and processing.
Hearing aids run on tiny voltages (typically ~1.4 Volts). They are designed to amplify speech (a small frequency range) at moderate volumes. An IEM is designed to handle the massive energy of a live drum kit without distortion. To do this, the internal amplifier needs Headroom. It likely steps up the battery voltage significantly (internally converting to higher voltage rails) to ensure that when a snare drum hits 120dB, the amplifier has enough electrical height to reproduce that spike without clipping.
Hearing aids use microscopic balanced armature receivers that require almost zero power to move because they are only moving a tiny amount of air near your eardrum. IEMs use dual-driver miniature subwoofers and tweeters that are physically larger and heavier. It takes significantly more electrical current to push these drivers back and forth.
Hearing aids often use aggressive battery-saving tricks, such as lowering the sampling rate or "sleeping" processes when silence is detected. The processor of an IEM is running wide open 100% of the time. It is constantly digitizing the world at a high sampling rate to ensure zero latency. If it tried to save battery by "sleeping" between notes, you would hear a delay (latency), which would make it impossible to play in time.