The short answer is it was illegal and would have been terribly unprofitable even if it wasn't. For a longer answer, consult this topic and this topic where similar questions were asked.

/u/SomebodyReasonable posted this, which links to several accounts from former slaves of slave fights for their masters' amusement.

John Finnely, for example:

De n----r fights an more for de white folks' joyment but de slaves am 'lowed to see it. De massas of plantations match dere n----rs 'cording to size and bet on dem. ... Dat fight am held at night by de pine torch light. A ring am made by de folks standin' 'round in de circle. Deye 'lowed to do anything with dey hands and head and teeth. Nothin' barred 'cept de knife and de club.

He described a fight between one of his master's slaves, Tom, who was undefeated, and a new opponent. It sounds brutal:

dat new n----r gits Tom in de stomach with he knee and a lick side de jaw at de same time and down go Tom and de udder n----r jumps on him with both feets, dem straddle him and hits with right, left, right, left, right side Tom's head. Dere Tom lay, makin' no 'sistence. ... Well, dat new n---r 'laxes for to git he wind and den Tom, quick like de flash, flips him off and jump to he feet and befo' dat new n----r could git to he feet, Tom kicks him in de stomach, 'gain and 'gain. Dat n----r's body start to quaver and he massa say, 'Dat 'nough'

Wiley Childress (the History Banter site says it was Cecila Chappel, but that seems to be a mistake), another former slave, [relayed](http://memory.loc.gov/cgi-bin/ampage? collId=mesn&fileName=150/mesn150.db&recNum=11&tempFile=./temp/~ammem_1LLD&filecode=mesn&next_filecode=mesn&prev_filecode=mesn&itemnum=2&ndocs=100) a story he heard in about men who stopped a runaway slave betting that no other slave could beat him. A slave pitted against him was killed in the fight:

De only story dat I member mah people told me 'bout wuz on Fedd, a slave on de next plantation. He wuz a big man en wuz de strongest man neah dat part ob de kuntry. He wouldn' 'low nobody ter whup 'in. De Marster framed 'im by tellin' 'im ter bring his saddle hoss en w'en hu hum wid de hoss several men 'peahrd en tole Fedd dat dey wuz gonna whup 'im. He struck one ob de mans so hahd dey had ter hab de doctuh. De Marster said let 'im 'lone he's too strong ter be whup'd. I'll hab ter shoot 'im. One time Fedd run 'way en de white men whar he stopped know'd he wuz a good fighter en made a $250.00 bet dat nobody could lick 'im. A n----r fum de iron wuks fought Fedd en Fedd won. De iron wuks n----r wuz kikt right dere.

It seems like arranged fights between slaves did, at least occasionally, happen. I don't think Django Unchained portrays it as something that was common—Candyland is supposed to be an unusually really, really terrible place.

ETA: another account, not of fighting, but to help illustrate that slave owners did not always do what made economic sense when it came to their slaves:

I will never forget how my marster beat a pore old woman so she couldn't even get up. And 'cause she couldn't get up when he told her to, he hit her in the head with a long piece of iron and broke her skull. Then he made one of the other slaves take her to the jail. She suffered in jail all night, and the jailer heard her moanin' and groanin', so the next mornin' he made marster come and get her. He was so mad 'cause he had to take her out of jail that he had water pumped into her skull just as soon as he got back home. Then he dropped her down in a field and she died 'fore night. That was a sad time. You saw your own folk killed and couldn't say a word 'bout it; if you did you would be beat and sometimes killed too.

A man in callin' distance from our place had a whippin' pole. This man was just as mean as he could be. ... A woman on his place had twins and she warn't strong from the beginnin'. The day after the chillun was borned, he told her to go over to his house and scrub it from front to back. She went over to the house and scrubbed two rooms and was so sick she had to lay down on the floor and rest awhile. ... [The master's wife secretly has the sick slave carried back to her home and has another slave finish] ... He got mad and said his wife was cloakin' for the slaves, that there was nothin' wrong with the woman, she was just lazy. He beat his wife, then went out and tied the pore colored woman to a whippin' pole and beat her unmerciful. He left her hangin' on the pole and went to church. When he got back she was dead. He had the slaves take her down and bury her in a box. He said that laziness had killed her and that she warn't worth the box she was buried in. The babies died the next day and he said he was glad of it 'cause they would grow up lazy just like their mother.

My marster had a barrel with nails drove in it that he would put you in when he couldn't think of nothin' else mean enough to do. He would put you in this barrel and role it down a hill. When you got out you would ben in a bad fix, but he didn't care. Sometimes he rolled the barrel in the river and drowned his slaves.

Just developing the question: How much does the trope of "Mandingo fighting" owe to the popular "Falconhurst" series/franchise sex and slavery novels started by Kyle Onstott with the novel "Mandingo" published about 1958. Black prizefights to the death do appear in the series and I assume that this is where the term comes from. Was it something that was regularly mentioned before the novels came out?