It's not only Mediatek. Both Samsung and Qualcomm are having 8 core SoCs on mobile too. And they all sell very well.
There's 8 of the same low end and there's 4+4 big.LITTLE, both are addressing different problems. You don't see big.LITTLE on desktops or even laptops because there isn't as much of a pressing need to actively use CPU cores down to below 200mW per core. And if you do, Intel has Atom which has better dynamic range into low MHz than something like Cortex-A57.
Ignoring big.LITTLE, Qualcomm throws 8 lower-middle end cores on an SoC because MediaTek does and started doing it before them. I can all but guarantee you they wouldn't otherwise. And they can do it because the cores they're adding are very small and cheap, increasing the overall SoC size by only a small amount, and are paired with little extra L2 cache to boot. AFAIK Samsung hasn't followed this trend. And even some of these still follow the big.LITTLE philosophy with a lower clocked cluster paired with a higher clocked one.
I think one of the main reasons we're not seeing 8 cores on desktop despite having gone from 65 nm (Q6600) -> 14 nm is that Intel does not want to cannibalize on it's high margin server CPUs. So there are simply no mainstream 8 core Intel CPUs available on the market.
Intel cores are big (much, much bigger than Cortex-A53), putting 8 or even 6 of them on a mainstream consumer chip results in a much larger and more expensive die, which is not a cheap value proposition for something most users don't benefit from. So why bother?
Intel already sells a class of server CPUs that are the same die as the desktop ones.. the statement basically boils down to "Intel doesn't want to sell 8 core chips because Intel doesn't want to cannibalize their 8 core desktop chips." The fact is that they DO sell 8 core chips for desktop enthusiasts, they just charge a lot of money for them, because they can. That would be the same if they made an 8 core desktop chip that ran on the same socket that the mainstream desktop chips run on, only it'd be substantially more expensive because it'd have to absorb the costs of making yet another die for a product few people want.
QCOM only has 8 cores cause they got caught with their pants down on the 64b transition, prior to needing to used A53/57 to fill in, they we maxed at 4 cores with krait, and its fully expected that they'll be back to 4 cores with kyro/kryo.
Kryo. It's just Kryo. There's no ambiguity. Qualcomm has officially named it.