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Best explanation I've ran across to date.

No matter what the media wants you to believe, It’s already a two-man race. Of the 2,286 total Republican delegates, 1,144 are needed to win nomination. In five States: Virginia, Missouri, Ohio, Tennessee and Illinois, Gingrich and Santorum are not even on the ballot. That’s 564 delegates that they cannot get. After South Carolina, only 59 delegates have been allocated to the 4 candidates. Mitt Romney has 39 of those. This means Santorum or Gingrich has to pick up 1,144 delegates from the remaining 1,683. Let’s say that Paul only picks up 20% of the total number of delegates–I happen to believe he will do much better–but let’s pick 20%. That’s 457 delegates for Paul. Add that to the 564 that Newt and Santorum won’t have a shot at and now there are 1,021 delegates that Santorum and Gingrich have no chance of getting. Subtract that 1,021 from the total 2,286 and you’re left with 1,265 delegates. That means that Gingrich and Santorum, one or the other, have to pick up over 90% of the available delegates to get the nomination.