Fortunately, the Motorola team have given some insight on why the update has been delayed for its former flagship.
In response to a tweet from a customer, the Motorola Support team revealed that the customized dual-core Snapdragon S4 Pro chip on the original Moto X was the cause of the delay.
The tweet says that the OG Motorola Moto X will get "L" (Android 5.0), but it is taking more effort on Motorola's part. There is no word from Motorola on when the update will be ready, but the important thing is that the manufacturer did say that it will eventually arrive.
@ac2k9 Moto X (1st Gen) will get L, but chipset requires more effort. We're working to deliver satisfactory experience, performance.
— Motorola Support (@MotorolaSupport) February 9, 2015
source - Motorola (Twitter)