Donald Trump is dominating the Republican polls, both nationally and state-by-state, but is still under 50% to get the nomination. Hillary Clinton is dominating the national Democratic polls, but is in a tight state-by-state battle with Bernie Sanders; she is 81% to get the nomination. Why the difference?
1) Trump is 37% nationally to 17% for Ted Cruz. This is a 20 pp lead, but still puts him 13 pp shy of 50%. Clinton is leading 52% to 36%. This is just a 16 pp lead, but puts her above 50%.
2) Trump has three to four viable alternatives while Clinton has just one. While the scrum of competitors is actually help Trump, because no one is running negative ads against him. They are too busy fighting each other. At some point in the future there will be just one competitor. If it is Cruz, then he is going to win the nomination. But, if it is anyone else, than he will finally get the brunt of negative advertising, which could hurt. As for Clinton, she just needs to defeat one challenger.
3) The Democratic rules actually favor Clinton more than the Republican rules favor Trump. The Democrats have a lot of “super delegates” not chosen in the primary, but the party establishment. They are heavily pledged towards Clinton. And, the Democratic National Committee has done everything else it can with its rules to help Clinton, including burying the debates when no one would watch them.