Because it’s still in beta. We’re not likely to get b2 this week, meaning that next wee we would get it. On that basis, you’re expecting 2 betas (b1 is buggy as hell, worse than the iOS 13.0 betas) so it needs 4 betas, minimum.
Only 1 iOS x.1 release only had 2 betas. It makes no sense they’d have iOS 13.1 ready for next week/week after.
I would disregard the number of betas for 3.1 - Apple are likely many builds ahead of beta 1 already. Regardless of our feedback 3.1 is over 250 (can’t recall the exact figure) builds ahead of the last 3.0 beta we got and would have added things they don’t want us to see until after the keynote. Time will tell - we can take this up next week and prove me wrong (or right).