In short
Many tax collectors and sinners came to recline and eat with Jesus and his disciples, because the occasion was a farewell dinner, since Matthew was leaving everything to follow Jesus (Luke 5:27), so he invited his circle of friends—tax collectors—to bid him farewell.
Jesus had just called the tax collector Matthew to leave everything behind and follow him. The dinner at his home that evening was therefore a farewell dinner, for he was leaving his job as a tax collector as well as his circle of friends at the time. This is of course why there were so many tax collectors and other sinners present.1
10 And as Jesus reclined at table in the house, behold, many tax collectors and sinners came and were reclining with Jesus and his disciples.