#2 Lack of other credible heels in the current roster
The fact is that there just aren't enough superstar heels in the roster right now. You have strong babyfaces like The Shield, but they have to contest against the likes of makeshift teams like Corbin, Lashley and McIntyre, who're just bodies put together to be fed to the Hounds of Justice. SmackDown Live has Daniel Bryan, but that's pretty much it.
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Many of the heels in the roster do not serve as forces of evil themselves but as footsoldiers for their highly evil overlords- the McMahon family. And this probably stems from both a lack of experience, to make the WWE Universe detest you and a lack of personality, in some cases. Character work has been secondary to in-ring work, in this era.
This is why The McMahons are forced to be on-screen heels. To their credit, they do a good job.