DiogenesLamp: "...he would have won the Chicago convention but for Lincoln's dirty tricks, and he would have been far less likely to have caused a civil war."Lincoln's "dirty tricks" were no "dirtier" than Sewards' own, but they worked better because Lincoln had stronger support.
And the basic reason is that Senator Seward at the time was most famous for his "irrepressible conflict" speech in October, 1858.
It was a rare occasion of a slick politician speaking unpopular truth, and it made Seward anathema to both Southern slaveholders and Northern Doughfaced Southern sympathizers.
That's why Republicans in Chicago turned to Lincoln instead.
