Chaos Knight

Chaos Strike

Critical Chance increased to 15%

Because why the heck does CK have a weaker version of WK's crit. If you're saying Void and Slardar have the same bash, Void's does magical damage and Slardar's does physical damage.

With his illusions, he got actually a 40% crit chance.

Make it 15% and he will have atleast one crit in 9 hits from all illusions.

CK's crit has to be weaker than other crits because his illusions get the ability.

To clarify, all illusions get crit abilities and will crit using their own damage(They use fake crit numbers to prevent them from being too obvious).
However, CK has illusions that do 100% damage, and that in itself is sort of like a 400% damage boost(Remember illusions don't benifit from raw damage). If you buffed his crit, that number would become even higher, making the hero possibly OP.
TL;DR CK already has super ilusions, so he can't get a more powerful crit for balance reasons.