What you call "All-App Multi-Tasking" is idiotic to put into a mobile device, for the following reasons:
1. Drain on resources - battery, CPU, and memory.
2. Poor (overall) performance - see #1
3. Due to the UI on mobile devices, not all apps need to be multitasking.
For those apps which need to be multitasking, ios allows those apps access to multitasking services to handle background processing, streaming, etc. Between ios multitasking services, and notifications, battery, CPU, and memory usage are minimized.
That's why IOS devices get excellent battery life and android devices don't.