The NYT article on a potential Trump/Pence ticket states:
The delay has the potential to complicate a partnership with Mr. Pence, who must file papers in Indiana by noon on Friday withdrawing from his re-election campaign in order for Republicans to field a new candidate for the race.
Why is this the case? Joe Biden, for example, ran parallel campaigns in 2008, and if Obama had not been elected, he would have (presumably) remained in the Senate.