Why are smartphones and tablets ARM based?
The main advantage to ARM is the power consumption. ARM is very efficient compared to other architectures. At the most basic level, the lower power consumption comes from the processor's instruction set: ARM devices use a RISC instruction set, which is a small but optimized set of instructions. This allows for fewer transistors and instructions, thus saving both power and space. As you can imagine, mobile phone manufacturers want to get the most bang for their buck when it comes to battery life. 

It's been widely rumored that Microsoft is going to move their laptop focus to ARM, but the trend has yet to take off in my opinion. As of right now, Intel processors pack a lot more performance punch at the cost of higher power consumption. This is a tradeoff manufacturers are willing to make with desktops and laptops, as it's usually relatively easy to supply more power to these devices (versus, say, a smartphone).