Because:
1. People worry. And if there is little indication of the 'next step' on the website, people specifically worry that their purchase has been successful.
2. Normally, people would expect the tickets to be ready to print immediately. It's 2015, and this has become the norm.
3. We have...
Blimey.
I ordered mine before 4pm and the email came through after midnight. I thought that was a bit silly - but clearly I got the express service.
At least you have the yespay email receipt, and thee game is two weeks away, so no need to worry - clearly you will get your tickets.