GTC Biotherapeutics (fka GTCB)

[used to make chicken]

The short answer is time.

GTCB and Pharming have been around a lot longer than any of the serious avian endeavors, which have been at it for about ten years. VRA doesn't count, because they are just throwing money at Roslin in hopes of slavation.

Chickens had some technical hurdles to overcome. Unlike mammals or fish or everything else, the chicken egg does not have a single pronucleus that you can inject. It has many, only one of which ends up being the correct one, so generating the initial mosaic transgenic is more of a numbers game. The primary obstacle however was finding a delivery system that would both 1) get the DNA of interest into the genome, and 2) do so in a way in which it was not "silenced."
Many genomes have ways of recognizing foriegn DNA and preventing any genes therein from expressing themselves. Lentiviruses were the ticket, and this has only been known for about 5 years.