@ Geremia, you should really abstain from citing a well-known anti-semite Bjerknes when trying to be objective about Einstein.
And no, Special Relativity doesn't "belong" to Poincare and Lorentz but to Einstein. (And it's ludicrous to talk about SR without acknowledging Fitzgerald and Maxwell). Your argument is tantamount to crediting Newton with the full invention of calculus when the heavy lifting had already been done by Descartes and Fermat on analytical geometry - a curiousa that explains how Leibniz was able to derive calculus almost simultaneously with the great Sir Isaac. In fact, the Bernoullis - and even Lagrange - always acknowledged calculus as the birth child of Fermat. So if we aren't going to split hairs on an issue such as that, then why not simply give Einstein his due for Special Relativity (which in a real epistemic sense is different than both Lorentz and Poincare's conceptions of 'relativity'). In theory, Einstein didn't actually need his scientific predecessors to create SR given that the Lorentz transformations naturally arise out of Maxwell's equations. If we're being super picky, then let's give Lorentz, Poincare, Fitzgerald, and Maxwell 50% of the credit, and Einstein the other 50%. But why stop there? Give Laplace 30% of the credit for celestial mechanics. Max Born should get 40% of the credit for Matrix Mechanics too. Robert Hooke should get 50% of the credit for coming up with the inverse square law which Newton subsequently stole and then mathematized. (I hope you get my point - you run into a fair bit of troubling when delineating attributions of scientific discoveries).
At any rate, regarding your question, there are a few reasons.
The nobel committee didn't fully understand General Relativity nor was there an immediate realization of how important GR was to the modelling of the universe until Wheeler and company advocated for it later in the century.
The nobel committee didn't fully realize the universality of SR until Dirac.
Einstein really didn't care for the Nobel prize and the Nobel committee took offense. On the week he was about to be awarded the prize, he received a message suggesting he stay around "for something big in Stockholm" (wink, wink, hint, hint). However, he had already planned a trip to Japan that week and didn't care enough about the Nobel to delay his trip (the Nobel committee certainly took offense). Not only did he skip his own Nobel ceremony, but he didn't even WRITE his reception of the award in his journal the day he received it. Later quotes by journalists asking why he never received more than one Nobel mirror his general indiffernce to the award. His only use of the Nobel prize was to complete a promise to his then ex-wife Mileva Maric that, in exchange for a divorce, he would give her the Nobel Prize money once he inevitably got it for one of the many brilliant things he did.
Much of Einstein's work was so revolutionary that there was a sense in which the committee members believed them to be too speculative. Plus, the committee has generally prejudiced experimentalists over theorists - but even that isn't enough to account for why they've given out multiple nobels to other scientists whose work were nowhere near as fundamental as Einstein's.
There's a great argument made by a plethora of science historians than Einstein deserved anywhere from 4 to 12 Nobel Prizes (klien wrote a great article on this a few years back). Stone: http://www.huffingtonpost.com/a-douglas-ston/einstein-fantasy-physics_b_4948045.html
Count the amount of Nobels given for Brownian Motion. Bose-Einstein Condensates. Special Relativity. General Relativity. His 1909 and, later, his 1917 Paper on the Spontaneous and Stimulated Emission, and many others. Heck, even the EPR paper would be worthy contender.