The physical factors behind the reduced Diels-Alder reactivity of the Sc3N@C78 metallofullerene as compared with free C78 have been investigated in detail by means of computational tools. To this end, the reactions between 1,3-butadiene and free C78 and endohedral Sc3N@C78 have been analysed in terms of regioselectivity and reactivity by using the activation strain model of reactivity in combination with the energy decomposition analysis method. Additional factors such as the molecular orbital overlap or the aromaticity of the corresponding transition states have been also explored. Our results indicate that the lower reactivity of the metallofullerene finds its origin mainly in the less stabilizing interaction between the deformed reactants along the reaction coordinate induced by the triscandium nitride moiety. Free versus encapsulated: The encapsulation of Sc3N into a C78 cage induces dramatic changes in both the exohedral Diels-Alder reactivity and the regioselectivity as compared with free C78 (see figure). By means of the recently introduced activation strain model of reactivity in combination with the energy decomposition analysis method, the origin of these reactivity/regioselectivity changes is analysed in detail ; We are grateful for financial support from the Spanish MINECO-FEDER (Grants CTQ2013-44303-P and CTQ2014-51912-REDC to I.F. and CTQ2011-23156/BQU to M.S.), Catalan DIUE (projects 2014SGR931 and XRQTC to M.S.), the FEDER fund for the grant UNGI08-4E-003 to M.S., the National Research School Combination: Catalysis (NRSC-C) and The Netherlands Organization for Scientific Research (NWO/CW). The ORFEO-CINQA network is also acknowledged. Support for the research of M.S. was received through the ICREA Academia 2014 prize of the Catalan DIUE