There are some huge differences between the regions in Spain. 14 day rate for the Canary islands is about 5.8 and South Tenerife has not had any positive cases for over 30 days. There has been no deaths in the Canary islands for 6 weeks. The Spanish PM has ruled out a second national lockdown and would rather let individual regions impose restrictions as they see fit.
Asking returnees from the islands (Canary & Balearics) to quarantine is not based on the science/medical facts. Unfortunately the UK Government is afraid to admit they got it wrong and will probably wait a week or two to safe face before reversing their decision.