Because the Jews represented both the best and the worst that humanity had to offer.
(In some ways, they do even now.)
In their capacity as best, God greatly blessed them; in their capacity as worst, God reviled them and fucked them in the ass with abject contempt as if they were filth.
This pattern is present in both the Old and New Testaments. An ironic thing is that in the OT the Lord brutalized the Israelites for straying from strict monotheism; in the NT Jesus condemns the Israelites precisely for their unsophisticated monotheism.
These qualities are simply two sides of the same coin. God could not make some of the Jews best without allowing the possibility that some of them would be worst. Moses and the prophets, Mary, Jesus, Jesus’ disciples were best; Judas the betrayer, the mob which demanded Jesus’ death, the ruthless Pharisees were worst. Uplifting some Jews toward glory entailed paying the price that some of them would be severely shamed.
A second answer is that OT to NT is as nature to grace. The law given to the nation of Israel was meant to purify their nature, and the prophets were raised to call Israel back to righteousness. The savage punishments served as incentives to the Jews to heed the law. Jesus came with the gift of sanctifying grace that could raise men above their nature. Since grace does not violate but builds on nature, anyone in the state of grace is spiritually uncorrupt and automatically immune from penance.