Because they're usually based on mobile phone or similar SoCs. However, at least 4 companies are promising to deliver Server-class chips this year - Cavium, Ava

Because they're usually based on mobile phone or similar SoCs. However, at least 4 companies are promising to deliver Server-class chips this year - Cavium, Avago (ex Broadcom), Qualcomm, and AMD. (Edited to add) Incidentally, most of the above are designed to have more than 8 cores. Cavium's ThunderX has 48, for example. Both Cavium and Broadcom previously delivered many-core MIPs designes, so this isn't vaporware either.