fly_high I believe it is simply down to cost. The RAF kept the Vulcan display going for about 2 years after the ac left service until it was out of fatigue.
I think the cost of the major servicing was IRO £1m at the time. BBMF OTOH has a significant amount of good will sponsorship and Spitfire bits are relatively easy to come by.
In addition to the simple operating cost, as a State aircraft, there would have been a significant cost associated with crew currency and groundcrew.