Here is the Orignal post made by administrator of Asus support forms
Asus was scheduled to provide ZenFone4, ZenFone5,ZenFone6 and PadFoneS with Android L upgrade in the end of April.We've sent the new version for final verification and we expect to release the upgrade soon.
Please refer to following table for new OTA plan. We would request the kind consideration and understanding of our customers with regards to any inconvenience caused by this schedule changing.
Q:Peoples are saying that Samsung,Motorola,Micromax brands are already Have lollipop updates on there devices. why asus so late??
Ans: Asus is doing a lots of Modification in there ZenUI so users can easily use the device and it can Look better then other brands.
Now the day comes today "15-05-2015" mid of may , but no respond from ASUS .
They may be have some planing behind the delay . No body knows
Let hope asus will release the update as soon as possible.