Pay, benefits and reputation are the main reasons.
Cons
Not all folks in analytics functions have any analytics background
Hard to feel tied to mission
Hiring too rapidly and bloating the organization (Zuckerberg recently addressed this it appears)
-Great Benefits
-Brilliant People
-Good Pay
-Others are willing to help and teach
Cons
-Processes do not scale which makes projects difficult to complete in a timely manner
-Stock going down has caused a lot of people to jump ship
-WLB is poor