While I can't speak to why other buy via iBooks, I have made exactly one iBooks purchase which was for a title that was not available in Kindle format. Everything else I get is in Kindle format that gets the DRM stripped and is then put back into iBooks because I prefer it to the Kindle app for reading. I have no idea why some books come out in iBooks first, but what was a non-issue at the time is now a minor annoyance.
While I can't speak to why other buy via iBooks, I have made exactly one iBooks purchase which was for a title that was not available in Kindle format. Everything else I get is in Kindle format that gets the DRM stripped and is then put back into iBooks because I prefer it to the Kindle app for reading. I have no idea why some books come out in iBooks first, but what was a non-issue at the time is now a minor annoyance.