Here are a few reasons. (I don't completely agree with all of them, btw, and some of them overlap a bit.)
- No sparring or 'alive' training.
- Limited technical curriculum (no body-to-body grappling, no punching/kicking).
- No aiki.
- Aikido has not proven itself in MMA fights.
- Developed from a very limited paradigm (koryu jujutsu / yawara) and never transcended that.
- Assumes the presence of weapons, but rarely trains with weapons present.
- Aikido makes unrealistic claims about doing a minimal amount of damage to the attacker.
- Most Aikido practitioners don't look mean enough.
- Lack of physical conditioning (strength/endurance/speed).
- Knowingly practicing many unrealistic (too elaborate) techniques to explore principles and ideas.
- Training techniques on poorly executed attacks during training.
- Aikido doesn't look violent enough.
- Training atmosphere is too relaxed, loose, fun.
- Lack of martial intent during practice.
- Practitioners with unrealistic ideas about fighting and self-defense.