The ancient Greek texts of these verses contain variations that have resulted in different translations. The NASB has the older son saying yes but doing nothing. The younger son says no but repents and goes. The younger son does the father"s will. The NIV has the older son saying no but then repenting and going. The younger son says yes but does not go. The older son does the father"s will. Probably the interpretation of the parable influenced early copyists. The better reading appears to be the one represented in the NASB. [1][source]
This is the first time Jesus applied one of His parables directly to Israel"s leaders ( Matthew 21:31). He introduced this application with His usual solemn introduction (cf. Matthew 5:16; et al.). Both the NASB and the NIV have translated the last verb in this sentence poorly. The Greek verb proago ("get into ... before" or "entering ... ahead of") here means "enter instead of." [2][source]
The tax gatherers and harlots were the dregs of Jewish society. Jesus undoubtedly shocked His listeners when He made this statement. The scum of society, though it originally said no to God, repented at the preaching of John and Jesus and thereby did God"s will (cf. Matthew 8:11-12). Consequently these people would enter the kingdom (by resurrection). However the religious leaders affirmed their willingness to do God"s will but refused to do so by rejecting Jesus. They would not enter the kingdom. [source][source][source]
Note that Jesus described both groups as sons of the father in the parable. All the Jews, those with a privileged position and those with none, enjoyed being sons of God in the sense that God had chosen Israel as His son (cf. Hosea 11:1). The leaders could still believe in Jesus and enter the kingdom. Individual salvation was still possible even though national rejection was strong. [source][source][source]