Because one side wants to treat the underlying causes, which it sees as poverty, inequality, lack of good education, and lack of opportunities; while the other side has the attitude of "fuck 'em, they're criminals", and wants to be "tough on crime" by increasing policing, criminal penalties, and the number of people thrown in jail.
The former side sees the justice system as fundamentally unjust and favoring the privileged, connected, and wealthy, and as discriminating against the poor.. as disproportionately affecting blue collar criminals while giving white collar criminals a slap on the wrist (if that).
Jail is seen as both a finishing school for criminals and a human rights disaster, where inmates are not actually reformed but come out as more tied in to the criminal underworld than ever, and where after they are released society almost forces them in to reoffending because legal work opportunities for those with a criminal record are so limited and many inmates learn nothing in jail except how to commit crimes more effectively.
Many see the police as abusive and corrupt, with the recent exposes on police brutality, murder, and corruption just throwing fuel on the fire. Police cultures which try to cover up abuses within their own organizations instead of bringing them to light make the general public even more distrusting. Giving more funding to police departments which betrayed the trust of their communities is seen by some as funding a criminal organization.
Meanwhile, the other side sees those police who are caught abusing their power as just a few bad apples (if that... some will support the police no matter what crimes they commit), and deny that there are systemic issues which need to be addressed. For some of these it's always the criminal's fault, if someone's in jail they deserve it, and the justice system is in fact fair and just.
That's not to mention vigilante, militia, and "patriot" groups which are happy to take the law in to their own hands to fight those they see as enemies.
Attitudes towards drug offenses (which are the reason a huge portion of people are in jail in America) also matter.
These are all very different views of the world, and those are some reasons why criminal justice reform and what should be done with or about police is so controversial.