Because we have a House of Representatives and a Senate which comprise Congress. Both work independently of each other, and each can be ruled by either party. The clincher is that both the House and Senate need to agree to pass a bill. If a bill doesn't make it thru both sides it dies. The deadlock we've been seeing for the past few years has come from the Republicans ruling the House of Representatives and refusing to pass any Democrat-proposed bill coming from the Senate, even tho the Democrats ruled there, and vice-versa.
Now that both sides are Republican I don't expect anything to change really. Since they are comprised of at least 4 warring factions amongst themselves (the tea party, moderates, extreme right wing faux-christians, and libertarians) it's highly unlikely both sides of Congress will ever agree on anything, and even if they do Obama has the power of veto to block any legislation that might successfully come out of Congress. Yes, Congress can override a veto with a 2/3rd majority vote, but that's extremely unlikely to ever happen. So essentially we will have another 2 years where absolutely nothing gets done.