BIS is no longer in play, so it's capabilities are now moot. As you recall, for non-PUSH email systems, BIS would poll the server every 15 minutes, and shorten that interval whenever there was something found on the server, slowly lengthening the interval back out to 15 minutes for as long as nothing was found on the server.
With the BB10, it's all ActiveSync directly between the device and the email service provider. If the service provider supports PUSH, you can configure the BB to use it, and it indeed works. If the service provider does not support PUSH, then you are limited to 15 minutes as the shortest interval available.
Typically, it's a matter of properly configuring no the device. For PUSH-capable email services, it often means bypassing the wizard configuration on the device and instead using the Advanced settings.
Lastly...as to your actual question..."why"...well, the only answer is because that's they way the programmed it. As to why they chose to program it that way, an answer would require access to proprietary "internal" decision making information...not very likely.
Occam's Razor nearly always applies when troubleshooting technology issues!
If anyone has been helpful to you, please show your appreciation by clicking the button inside of their post. Please click here and read, along with the threads to which it links, for helpful information to guide you as you proceed. I always recommend that you treat your BlackBerry like any other computing device, including using a regular backup schedule...click here for an article with instructions.