Why Krishna generally appears in kshatriya families?

SRILA PRABHUPADA SAYS:

“The family in which Krsna appeared is called the Yadu dynasty. This Yadu dynasty belongs to the family descending from Soma, the god in the moon planet. There are two different ksatriya families of the royal order, one descending from the king of the moon planet and the other descending from the king of the sun planet. 

Whenever the Supreme  Personality of Godhead appears, He generally appears in a ksatriya family because He has to establish religious principles or the life of  righteousness. The ksatriya family is the protector of the human race, according to the Vedic system. 

When the Supreme Personality of Godhead appeared as Lord Ramacandra, He appeared in the family descending from the sun-god, known as Raghu-vamsa; and when He appeared as Lord Krsna, He did so in the family of Yadu-vamsa. There is a long list of the kings of the Yadu-vamsa in the Ninth Canto, Twenty-fourth Chapter, of Srimad-Bhagavatam. All of them were great, powerful kings.”

“Krsna's father Vasudeva, son of Surasena, descend from the Yadu dynasty.

Actually, the Supreme Personality of Godhead does not belong to any dynasty of this material world, but the family in which the Supreme Personality of Godhead appears becomes famous, by His grace. 

For example, sandalwood is produced in the states of Malaya. Sandalwood has its own qualifications apart from Malaya, but because, accidentally, this wood is mainly produced in the states of Malaya, it is known as Malayan sandalwood. Similarly, Krsna, the Supreme Personality of Godhead, belongs to everyone, but just as the sun rises from the east, although there are other directions from which it could rise, so, by His own choice, the Lord appears in a particular family, and that family becomes famous. 

When Krsna appears, all His plenary expansions also appear with  Him. Krsna appeared along with Balarama (Baladeva), who is known as His elder brother. Balarama is the origin of Sankarsana, of the quadruple expansion. Balarama is also the plenary expansion of Krsna.”