It's plenty competitive. However, it is not as aggressive-dominated nor as technically complex as Melee or PM, which some people mistakenly consider vital traits for a game to be "competitive".
In my opinion (and I think this is shared by many, though I could be wrong), Smash 4 has achieved a nice balance of offensive and defensive validity that is unusual in the series. It augments this variety with wider character viability (something we've not seen in the series, ever), and with (internet-dependent) good online play for practice, and a respectable amount of stage variety. Balance patches are the cherry on top, and all of that together makes this what I personally consider not only the most fun Smash Bros game, but the one with the most competitive potential (from a size-of-scene standpoint) as it accomplishes all of this with extremely few highly technical characters or, as mentioned above, virtually mandatory techniques.
So, to answer the question, "It is competitive, people who say otherwise are using a disappointingly narrow definition of competitive."