Since DisplayPort is superior and more or less a super-set of HDMI, I doubt Apple will bother with HDMI... ever. Once they enable audio through DP, its game over.
Plus DisplayPort is free as it's designed by VESA while HDMI costs and what I've heard, it's not even cheap. It's not a surprise that Apple chose the free one. DP is becoming more popular all the time as it's free so that means cheaper GPUs.
BTW, DP has maximum bandwidth of 17.2Gb/s + 720Mb/s while HDMI tops out at 10.2Gb/s