I guess this is the cause: mine is iMac, mid-2011:
Handoff and Instant Hotspot
Supported by the following Mac models:
- MacBook Air (2012 or later)
- MacBook Pro (2012 or later)
- iMac (2012 or later)
- Mac mini (2012 or later)
- Mac Pro (late 2013)
Handoff and Instant Hotspot work with iPhone, iPad, or iPod touch with iOS 8 and the Lightning connector.Phone Calling and SMSRequires iPhone with iOS 8 and a Mac with OS X Yosemite. Requires activated carrier plan.AirDropAirDrop between OS X and iOS is supported by the following Mac models:
- MacBook Air (2012 or later)
- MacBook Pro (2012 or later)
- iMac (2012 or later)
- Mac mini (2012 or later)
- Mac Pro (Late 2013)
AirDrop to iOS devices requires iPhone, iPad, or iPod touch with iOS 7 and the Lightning connector.AirDrop between two Mac computers is supported by the following Mac models:
- MacBook Pro (late 2008 or later)*
- MacBook Air (late 2010 or later)
- MacBook (late 2008 or later)*
- Mac (early 2009 or later)
- Mac mini (mid 2010 or later)
- Mac Pro (early 2009 with AirPort Extreme card, or mid 2010 or later)
* The MacBook Pro (17-inch, late 2008) and the white MacBook (late 2008) do not support AirDrop.