* Poor support for APIs used to perform compute on GPUs

* Poor area density. If you were to try mine with igpus, you need one system per igpu, and even if you just stacked motherboards and power supplies together in some sort of custom case, you clearly get a lot more compute per area with dgpus.

* Poor performance.

No doubt that they're poorer performance than discrete GPUs, but given the scarcity and cost of the latter I would think they would settle for integrated gpus rather than no gpus at all?

Mining isn't free. You need to pay for space and power (especially when we're talking about the large scale miners capable of making an impact on the hardware supply). With most bitcoin like coins, the hashrate of the network has diminishing returns as the difficulty increases and the amount of compute resources needed to mine blocks at a decent rate increases. If your computing capabilities are worse than your competitors, it's unlikely you'll make a profit (which is part of why bitcoin mining is the land of ASICs these days, gpu/cpu mining of bitcoin doesn't happen, most gpu miners mine ethereum or monero, then if they want bitcoin sell that and buy bitcoin).

Cost/benefit. The iGPUs are low hash, you only get one per complete system. With dGPUs, you can stack 3-4 into a single system, and each one of those is orders of magnitude more powerful than the iGPU.