What people fail to realize about Lumumba is that, while he had lofty goals, he was an incredibly incompetent politician.
His Independence speech, for example, is the prime example. While it might have been awesome for him to call out the King of Belgium for his patronizing nonsense, even many Congolese nationalists thought it was a stupid thing to do.
Remember, Congo was still Belgian-run, and many politicians, like Moise Tshombe, understood that working with Belgium was the best option.
Lumumba's speech was basically biting the hand that still fed Belgium.
Also, his attempt to escape from Mobutu's clutches was clumsy. Had he not paused to give a speech, he might've lived.
Lumumba may have been awesome, but what he did implies he lacked any political sense and might have mismanage the country anyways.