Please explain why does my temp crown keep falling off?

Temporary crown: There are a number of potential reasons: durability of cement, fit of temporary restoration, your diet, your hygiene methods( hopefully your dentist counseled you on the last two) , do you chew gum or eat candy( i hope you were advised against both).... Contact your dentist as the temporary crown holds 3 dimensional space , so keeping it in place is important

See your dentist: Sometimes temporary crowns fall off due to improper fit or lack of provisional cement. Your dentist may be able to re-cement temporary crown if not, a new temporary crown will need to be made. Take care.

Evaluate by dentist: Go back to the dentist, perhaps the tooth is too short and needs crown lengthening prior to the fabrication of the new crown. In general, with a temporary crown, you need to be careful, in terms of diet--ie-sticky foods that can dislodge the provisional.