The question flows from the assumption that Thomas was the ONLY disciple who was absent during the first apparition . What Jn 20:19 says is:
Then the same day at evening, being the first day of the week, when the doors were shut where the disciples were assembled for fear of the Jews, came Jesus and stood in the midst, and saith unto them, Peace be unto you.
The term 'disciple' applies to both the Apostles as well as to the 72, the group that may have included women disciples. Of course, the Apostles exclusively met in important meetings. We do not know how fast the news of the Resurrection reached all the 11 Apostles, some of whom may have been hiding in isolation, so as to enable them get together on Sunday evening. It is therefore, quite possible that apart from Thomas a couple of other Apostles were absent during the first apparition. Thomas stands out for the very reason that he straightaway refused to believe . The other Apostle(s) who were absent might have readily believed.