Why is Snowball so unpopular in Animal Farm?

Snowball is a popular character on the farm until he is attacked by Napoleon in Chapter Five and forced to flee. After this, Snowball becomes unpopular on the farm because he is vilified through Napoleon and Squealer's propaganda campaign. This targeted campaign begins soon after his departure when Squealer tells the other animals that Snowball is "no better than a criminal." When the other animals protest by arguing that he fought bravely at the Battle of the Cowshed, Squealer further blackens his character by stating that he lacks "loyalty and obedience" and quickly changes the subject to the return of Mr Jones. 
Similarly, Napoleon tries to increase his own popularity by further destroying Snowball's reputation. He tries, for instance, to take full credit for the invention of the windmill and claims that he prevented to be against it as a means of getting rid of Snowball who is a "dangerous character" and a "bad influence." With no opportunity to defend himself, Snowball's popularity plummets, leaving Napoleon free to continue his anti-Snowball campaign.