The zigbee spec is freely available, I fail to see how BLE would be any more "open source".
I think it's just that every smartphone, tablet, laptop, etc now has BLE integrated, so it's easy to make a widget like a soil moisture sensor and connect it to your phone. Also, the newer LE version of bluetooth is quite power efficient so it can run for a while on a CR2032 if you take care to put the MCU in sleep mode between measurements, and whatnot.
Obviously almost everyone has a device to receive BLE whereas zigbee is a bit more specialist.
I've been playing with some arduino hardware that supports zigbee as well as BLE, and BLE is trivially easy to get working where the zigbee stack is much more complicated and much less documentation. So I would guess that plays a part too.
