Because they were commissioned and paid to do so.
The Last Supper is the last meal Christ and his disciples had together.
Hundreds of artists through the ages have painted the 'Last Supper'.
If you mean Leonardo's painting (because lots of other artists have painted this subject) it was commissioned by the Duke of Milan.
last supper
A Renaissance painting.
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The Last Supper
The Last Supper
the last supper and The Vitruvian Man
Christian religious art.
In the famous painting "The Last Supper," it is supposed to depict Christ with His twelve disciples. They were celebrating the Jewish Passover seder, which was the last one before Christ's arrest by the Romans. It's been speculated in some circles that the person standing by Jesus might have been Mary of Magdalena.
The Last Supper = the last meal Jesus ate with his disciples was when Jesus was 33. The chronology is disputed, it might have been somewhere between 7 BC and 7 AD. So much for your question. But maybe you do not mean what you write. Maybe you want to know when the painting representing that Last Supper was painted. This is also difficult to answer, because this subject has been painted by hundreds of artists from the 3rd to the 20th centuries. So, maybe you believe that Leonardo da Vinci's painting 'The Last Supper' is the only one. That is a mistake. Anyway it was painted 1495-1497.