[BiseCreesia]

Salam

Is there is any compulsary to wear a cap ?

Not "compulsory" but recommended.
And in Salah it's makruh to pray bareheaded.

Is there is any ahadith which says us to wear cap compulsary ?

No, as it isn't compulsory, but it's a blessed Sunnah of the Prophet and there is a compulsion for the lover's hearts to follow his blessed Sunnah.

If it is compulsary then why some people do not wear cap in salah ?

Because they don't follow the sunnah in that.

Anyway what some people do isn't a proof, otherwise we may also ask:

"If Salah is compulsory then why some people don't pray Salah"?
answers me with only reference to quran and sunnah pls...