Burrington primary was removed because NATS could not afford a new head (radar!). There is no area primary cover west of the Pecos and that affects SAR. The computer systems, especially NAS, are not especially interested in primary returns. Primary is discarded when NAS is happy it has sumething that it can track. There will be two Burringtons from December (plan A) one will be a combined service and the other SSR only.
There may be other reasons why a new primary service is required but I don't know them. Unless its to cover aircraft with electrical failure that, if they lose their transponders dissapear!