Because Lincoln hirelings, specifically brought in on railroad cars through his connections with the railroad corporations, overwhelmed the convention and deliberately blocked access to Seward supporters.
Now which party buses in the Astroturf? Doesn't it start with a "D"?
Lincoln rose from 2nd place (after Seward) on the first ballot to first place on the third, after which a solid majority switched to Lincoln.
Sure, after conveying proposed bribes to sufficient delegates to get them to switch. Again, which party does that sound like?
Liberal corporate Lawyer from Illinois engaging in dirty tricks in the Corrupt city of Chicago? Sounds like a Democrat to me. Sounds like a fairly recent one in fact.
I think today all sides use whatever means necessary to get as many supporters as needed to important places, conventions, etc.
So here's the question you need to ask yourself, FRiend:
Seward was the big loser, had the biggest grudge to carry against Lincoln if he felt mistreated.
But how, in fact, did Seward come to view Lincoln?
I'd suggest to you that party leaders in Chicago, who knew both Seward and Lincoln well, understood that Seward would make the greatest possible Secretary of State and Lincoln the greatest President and so chose consciously & knowingly.
I believe they were wiser than you give them credit for.