Lots of personalities out there. Not all personalities are good, and plenty create talking points because of that.
Agree with hrotha that generally this often boils down to people wanting more a-holes, and preferably arrogant ones, hence the fact Riccò still gets talked about to this day, and why there are many fans that like Sagan and his "I don't care what I have to do to get attention, as long as people are looking at me" attitude. Nacer Bouhanni is a character, sure, but doesn't seem to inspire the same strong support as well as backlash. Cavendish a few years ago created the same divided opinion, between those who saw a cocky, brazen champion who was prepared to shoot from the hip, and those who saw an arrogant, entitled a-hole who needed bringing down a peg or two (especially when compared to the target of much of his abuse, André Greipel, who with a couple of exceptions conducted himself with far more class throughout their rivalry).
It also depends on what you want from "characters". Plenty of people just want people who make the sport more fun to watch, whether they be negative character types or positive ones. That's why you see strong opinions elicited over riders like Jens Voigt and Thomas Voeckler, while riders who have been far more successful than them see less discussion and attention unless their characterlessness becomes an actual defining feature that is the discussion point (such as with riders like Louis Meintjes this year). Vino is another example, a pantomime villain but for whom the intangible X-factor that he brought to the sport with his never-say-die attitude on the bike made him many fans simultaneously.
Ultimately, though, cycling is an endurance sport and a cast of thousands which doesn't have the global reach of football nor the high audience share in specific markets of the NFL. While teams can have central leaders that become their mouthpiece in the same way as, say, a quarterback in the NFL, you have to stand out among a pack of 120-200 at various races, not among a pack of 20 or so on the field at any one time. You aren't competing head to head with one adversary but in a battle royale with several, and as a result you also have the issue of not wanting to ruffle too many feathers in the péloton to prevent angering them to the point of riding against you, hence only those who are truly at the pinnacle are able to strut their peacock feathers like a Cipollini without paying for it in a loss of results (an example could be the responses from the péloton to Lizzie Deignan's eleventh-hour Olympic reprieve after a track record of blaming others at all times, which garnered an interesting response). A footballer doesn't need to be better than the entire division working together to deny them space on any given day, nor do 20 of the league's cornerbacks come together on a field at any one time to block a superstar quarterback's passing lanes. But a cyclist does have to run the risk of being chased down, wheel-sucked or boxed in by a number of adversaries simultaneously, sometimes simply for the single purpose of preventing their success.
You say, why can cycling not have so many characters as, say, football, but if you condense it down by numbers and look at the number of 'characters' as against the number of interchangeable central defenders, goons, guards and tight ends, I don't think you'd actually find the proportion is that much lower, it's just that sometimes some of the biggest characters don't get as much of a platform to shout about it or know better than to ruffle the wrong feathers.