Good question which has many plausible answers. For contractors it’s just part of the business. I find it humorous that many of my clients, when presented with a resume for someone that has been consulting, “worry” about their job history being so short.
You can sell your reasoning for the short durations with something as simple as you value your personal integrity. The positions you were hired for fell short of those goals and standards and you moved on. That you choose not to suffer through a position just for duration times listed on a resume. Time is money right? Wasting your time in a go no where position is wasting your money and we all only have so much time we are able to work before we age out of the workforce…so move on and find the right place for you.
Now be prepared to answer how these companies fell short of your goals with a counter question of how will this future / prospective employer meet these goals so that you can be the star they seek.
I hope this helps some. It can be tricky but life is all about perception. It may look negative to some people but when explained correctly it may become a strong positive that reflects well on your character and personal standards you set.