Summary: The reason is that we, the church, are those to whom the vineyard was given and we need to be committed to tenant work. We are now those to whom the vineyard is rented and God expects returns!
COMMITTED TO TENANT WORK
I. THE SITUATION
The situation that Jesus describes in the opening lines of his parable is probably not a situation that the people He is teaching are unfamiliar with. There have always been men who build and then move on. Owners of farms and vineyards have always hired help to do the work. The situation is not unusual.
The owner of the vineyard left these renters or tenants in charge of his vineyard. What were they to do? Verse 10 tells us that he sent a servant to collect some of the harvest. To me, this meant that all the while the tenants were living in the vineyard, they should have also been working the land and planting. They should have kept up the facilities that were entrusted to them. They should have managed the land and also should have known they were accountable to the owner. All of us at one time or another has rented something. We know that while we have what is rented we should treat it as if it were our own, even though it is not our own. Payments need to be made to ensure that we keep the rented item.
ILLUSTRATION... Renting a chain saw
I rented a chain saw recently when a tree fell in my yard after a storm. I went down to Action Equipment in town and rented a chain saw for the day. They have all kinds of equipment. I rented a chain saw for the day and signed an agreement stating several things. It stated that I would take care of the chain saw and that I was responsible for fuel. It stated that I would pay for severe damages to it when I returned. It stated the time I was to return the chain saw and the penalties if it was late. The chain saw was also supposed to be returned cleaned and in good order or a charge would be issued. None of those things seemed out of the ordinary to me and seemed normal when renting something. There are understood rules and payments when renting something and I believe the owner and these tenants had an understanding as well.
This story is about an owner who leaves his vineyard to tenants and goes away. He expects, while he is gone, for the vineyard to be worked and harvested as if he were there himself. Make sense? Who is this story about? What is Jesus talking about? Jesus is teaching this while in the temple courts (Luke 20:1-8 (quickview) ) and is addressing common people as well as the chief priests and elders of the people. These religious leaders knew the Scriptures and knew that Jesus was telling a story about Israel.
* Isaiah 5:1-7 (quickview)  = "I will sing for the one I love a song about his vineyard: My loved one had a vineyard on a fertile hillside. He dug it up and cleared it of stones and planted it with the choicest vines. He built a watchtower in it and cut out a winepress as well. Then he looked for a crop of good grapes, but it yielded only bad fruit. Now you dwellers in Jerusalem and men of Judah, judge between me and my vineyard. What more could have been done for my vineyard than I have done for it? When I looked for good grapes, why did it yield only bad? Now I will tell you what I am going to do to my vineyard: I will take away its hedge, and it will be destroyed; I will break down its wall, and it will be trampled. I will make it a wasteland, neither pruned nor cultivated, and briers and thorns will grow there. I will command the clouds not to rain on it." The vineyard of the LORD Almighty is the house of Israel, and the men of Judah are the garden of his delight. And he looked for justice, but saw bloodshed; for righteousness, but heard cries of distress."