Land Reform and slavery had an important part in the late Roman Republic because they are ones of the many factors that contributed for the Crisis of the Roman Republic and the advent of the Roman Empire in 134 BC.
The Roman Republic had many great military successes but economic failure, this made the number of slaves increase causing unemployment among the lower classes. This made space for populist ideas and the eventual fall of the Republic.
The land reform came in the late Republic because of land concentration. Gracchus reforms happened in the 2nd century but had little or no effect. In 15 years the lower classes were in the worst position than before and they were also reduced to unemployment because of the slaves (as said above).
Cicero also tried to fix the land issue with a land reform bill but the Senate voted it down.