Because Robespierre wasn't trying to be a dictator. The problem with a lot of 'popular' histories of the French Revolution-- especially ones that are on YouTube for example, or even a history of it you might receive from a Western Civ class or 101 class in college-- is that they try to cover such a long timeline that they must by necessity resort to sometimes wild oversimplifications. "Robespierre was a dictator and caused the Terror" is one such oversimplification-- and is untrue.

An interesting thing to note is that Robespierre's tarnished legacy isn't the invention of modern historians-- on the contrary the moment he had fallen all of the "odium" acquired by the Committee of Public Safety (as RR Palmer poetically puts it) was heaped onto Robespierre. He became a scapegoat in the most extreme sense, and men who were inextricably, directly, linked with atrocities in a way Robespierre wasn't, began to blame him for every bad thing that had befallen France in the past few years. We have a very fascinating journal entry from a British sea captain who was a prisoner on a French jail-boat during the Thermidorian reaction. In his accounting, he explicitly states that those who had committed great atrocities began to pass that blame to Robespierre, and account to him everything they were ashamed of having done.

Robespierre never tried to pursue 'dictatorial' powers. He was a man on the Committee of Public Safety which have broad, but not complete, executive power over France due to necessity. It often is downplayed, but France was in very dire straights in 1792-1794. The war with Austria and Prussia was going very badly for the French, and internal revolts threatened to destroy the unity of the country and the revolution itself. I have a few posts that go more into what the Committee's power limits were ( Could the French Revolution's Reign of Terror be described as a dictatorship of the Parisians instead of of the Committee of Public Safety? ) and some about Robespierre personally ( How much of a moderate influence was Robespierre during the Terror? & Could the French Revolution's Reign of Terror be described as a dictatorship of the Parisians instead of of the Committee of Public Safety? )

Lastly, Robespierre wasn't unpopular, in fact he was quite popular throughout France. What did become unpopular was the fact that after the war began to die down after June 1794, the Great Terror didn't stop. Robespierre and his close associates were brought down for a number of reasons, but first and foremost is the fact that the Thermidorians-- the name we give the anti-Robespierrists who brought him down-- were all men with something to fear. They were either corrupt, or had been far too lenient, or were extraordinarily ruthless. For one reason or another, these men believed they could be next to the guillotine, and so decided to bring down Robespierre before he could bring them down first.

To be clear none of this is to pass judgement on Robespierre's involvement in the Terror-- he was a very powerful man and certainly held real authority. However the shorthand of "Robespierre was a bloodthirsty tyrant" is a lazy attempt to distill an incredibly complex and vastly more interesting topic down into a few buzzwords that can be covered on one lecture slide. Robespierre was neither bloodthirsty, nor did he aim to be a tyrant. I would highly recommend reading through my linked posts if you're curious in more supporting info there, and please feel free to ask any more questions you might have!