Vectoring is probably a financial not technical decision.
They have made the business decision to roll out g.fast over the next few years to FTTC areas. To maximise the take-up of those g.fast services they need the performance gap to be as wide as possible, vectoring would reduce that gap. Also vectoring is potentially expensive if they only utilise per card vectoring which would require rearrangement of copper bundles to get optimal performance which in turn requires manpower.
I remember posting over a year ago I can forsee BT only rolling out new technologies to hauwei cabinets, only one person agreed with me and that was Ronski. We both recognised BT have a thick skin when it comes to dealing with bad press or unhappy consumers and they wouldnt even flinch at the though of creating a 2 tier service based on cabinet lottery.