From what I've been able to locate, the Nexus 7 mini-tablet is about the only tablet that can actually handle wireless charging at this time. I was also unable to find an answer to why this is, but I have some reasons to consider.

Before that, though, I did run across an article about a company called Freescale that was supposed to provide wireless charging capabilities to tablets as early as 2015. Trying to find anything regarding Freescale and if they actually did produce wireless chargers for tablets after that time did not turn up a lot of leads. It appears the company was bought out, and that may be why Freescale not only seems to disappear, but also may not have gone to market with their tablet wireless charging technology.

COST

Notice when I mentioned the Nexus 7 I said it is called a mini-tablet. In pictures, it doesn't look much wider than a standard smartphone. So, the cost to produce it, most likely, is less than it would be if it were larger.

Devices that can support wireless charging need to be made out of material that can wireless conduct electricity. Plastic or a metal finish like aluminum are not the best of conductors, especially when talking efficiency. Most people want to have their device power up in a short period of time.

Glass is more or less the standard material used for this kind of conductivity, which means it's not only used for the screen, but also for the back. This glass also needs to be tough, scratch resistance, etc., to handle the normal wear and tear of a device. All of these things will drive cost.

SIZE

You may have noticed that there are some devices in your car that you can charge through a USB port (if built in), or with an adapter via the cigarette lighter, while there are others you can't. Usually the size of the device and how much power it requires are the factors involved. Similarly, wireless chargers are mostly built for smaller devices such as a smartwatch or a smartphone. The amount of watts required to power a larger device is different than it is for a smaller device.

CONSUMER DEMAND

It could be that demand for the ability to wirelessly charge a tablet is low compared to what it is for a smaller device. Typically, a phone or watch will get more use than a tablet does for the normal user, and so the convenience of wireless charging is higher than it would be for a tablet.

IS IT TRULY WIRELESS?

As an aside, but perhaps pertinent to the conversation still, wireless charging isn't truly wireless. Not yet .You have a charging pad that itself must be plugged into an outlet to provide the charge, or else (though I'm not aware of one yet), be charged at one point through a wire of some kind so it can turn around and charge a device.

Regardless, the tether to an outlet or powered USB port still exists.

Another company I came across was working on using infrared (IR) to power multiple devices up to 30 feet away. As far as I know, that charger, with the ability to truly wirelessly charge, has not been brought to market, and even so, it's likely it would take awhile for compatible devices, including tablets, to appear.