Blame Intel. Their licensing terms mean that you either use an i-series CPU and discrete graphics (too hot/power hungry for the mini) or you use the Intel onboard GPU which is not that great, although SB is better. Without this issue we would have had i3 minis with even better integrated GPUs last year.
Apple products always look outdated as they approach the end of their lives as they are competing against dozens of other manufacturers who have much more varied update cycles. The competition can afford to update more frequently as well as they don't have to (and often don't care) about compatibility and software issues. Sometimes it also pays to wait. P67 recall anyone?
