Microsoft calls it "hybrid sleep"
Microsoft calls the feature "hybrid sleep". (Source.)
It's normally disabled on laptops, for good reasons
In an article, Raymond Chen writes that the feature is disabled by default on Windows laptops, for good reasons:
"Hybrid sleep is on by default for desktop systems but off by default on laptops. Why this choice?
"First of all, desktops are at higher risk of the power outage scenario wherein a loss of power (either due to a genuine power outage or simply unplugging the computer by mistake) causes all work in progress to be lost. Desktop computers typically don’t have a backup battery, so a loss of power means instant loss of sleep state. By comparison, laptop computers have a battery which can bridge across power outages.
"Furthermore, laptops have a safety against battery drain: When battery power gets dangerously low, it can perform an emergency hibernate.
"Laptop manufacturers also requested that hybrid sleep be off by default. They didn’t want the hard drive to be active for a long time while the system is suspending, because when users suspend a laptop, it’s often in the form of 'Close the lid, pick up the laptop from the desk, throw it into a bag, head out.' Performing large quantities of disk I/O at a moment when the computer is physically being jostled around increases the risk that one of those I/O’s will go bad. This pattern doesn’t exist for desktops: When you suspend a desktop computer, you just leave it there and let it do its thing."
You can override the default, in exceptional cases
Perhaps the reasons don't apply to you. For example, perhaps you're using a laptop which you never take anywhere, and which no longer has a working battery. If this is the case, you can enable "hybrid sleep" manually. Instructions are in the aforementioned article.