People like them because they sell nice looking products with features that are appealing to customers. Putting all personal opinion aside, features such as powered lift-away mode, the DuoClean heads, quick draw wands, anti-wrap technology, sealed systems, bagless bins, odor reduction, and so forth are all attractive features that Shark vacuum cleaners have among other features. I may be alone here on Vacuumland, but a lot of their more expensive vacuum cleaners are not awful to use and have features that I personally find to be useful, some of which are unique to Sharks. Not all of them are that nice to use, but some of them are not horrible.
With that said, they do get a reputation in the service industry because they have a few select problems; examples being the hoses ripping, the microswitches failing, the brush rolls failing (mainly the DuoClean ones), the cyclones being poor, and so forth. I agree with MadMan, though. These are not necessarily things that they need to fix, because a good chunk of their customer base keeps their vacuum cleaner purchases on a rotation anyway. Why dump money into revising the current designs and upping the build quality when that money can go to designing and releasing a slightly different and well-marketed new model that people will jump on buying? Plus, their reputation among the average consumer is fantastic, so they have that brand recognition and are a trusted name.
Some people also seem to have a fascination with hating Shark. They are not good vacuum cleaners for the "buy it for life" crowd, but I do not think they are trying to appeal to people who want to keep their vacuum cleaner for decades. Some people exaggerate the lifespan of a Shark and say that they normally last six months to a year, but a more realistic lifespan that I have found is usually two years to five or six years, although I have seen people make them last longer. The main thing that kills them is definitely the hoses, though. Like anything, it usually comes down to if they want to fix it or buy a new one, and customers often opt for the new one. I have noticed that certain Shark models tend to be tougher than others, too. Some of the more mid-range models they sell without the fancy heads, but with longer hoses tend to outlast the more basic ones and more complex ones, in my experience at least.
Those are just my thoughts.