Good question RM999,
I think, they didn't want to push their luck in this case, similar to what Phil said.
Vettel was in lap 29 ~6.6 sec ahead of Alonso ( which just had stoped), he then lost ~1.9 sec in the next 7 laps, which on average is ~0.27 sec a lap. If we extrapolate this degradation rate for 26 laps (from lap 29 to 55), he would have lost 7.05 sec
and was only 6.6 sec ahead in lap 29, so while Alonso would have needed to make the pass, I think it is fair to say, that he would have had a good chance on his much fresher tyres.
That does not take the 2nd SC into account and assumes that SV would not have hit the "Cliff", the chance to do so, was high I would say. Could have end in an Alonso Canada scenario, SV lost ~0.5 sec from lap 34-35, which may let RBR consider that they approach the "Cliff", and decided to bring him in.
I don't think, that in their situation this gamble would have been worth the risk, but if they would have pulled it off, it sure would have been a huge moral blow to Alonso/Ferrari.