*Larger marketshare (this alone is the primary reason as ITX remains a niche market)
*Better performance per watt on the CPU side
*"Good enough" GPU performance for GPU accelerating HD videos and light gaming
*Lower TDP (55W vs 65W)
*Often overrated their TDPs compared to AMD (Intel 55W < AMD 55W)
I'm sorry, but historically there has been a short supply of AMD ITX boards.