It's called due diligence. Someone like Trump who has done thousands of big deals knows it is basic to make sure you are dealing with someone who not only is who they say they are but who has authority to make a deal.
Trump initiated some basic background research and let it be known what he was doing. He raised the fact that obama has gone to extraordinary lengths to conceal every aspect of his past, not just his BC. For example, there is the issue of whether he applied to colleges as a foreign student. There is the question of howhe got into prestigious schools with what must have been a so-so record. There is the question of how someone who doensn't exactly give off sparks intellectually could graduate magna from Harvard Law. There is the ongoing question of whether the guy obama claimed he barely knew, Bill Ayers, wrote obama's first book.
Releasing the BC only after Trump started nosing around sends a signal to me that they wanted to foreclose any more snooping.
And of course, there is the unresolved issue of whether the child of a British father can ever satisfy the Constitutional requirement of being "natural" born.
The media will take its marching orders from the White House and pronounce any and all inquiries to be resolved, just like they did in 2008. And no doubt, anyone who raises awkward questions will be attacked as a racist.