Here are the highlights:

• The Eucharist is a fundamental ceremony in the christian religion so monasteries and bishops would be motivated and in some cases obligated to produce the needed wine.
• Monastic abbots and bishops were part of the aristocracy in the middle ages and they were socially obligated to be hospitable to other aristocrats when they had such guests. It was expected that this hospitality include the serving of quality wine so church officials had a large motivation to maintain vineyards.
• Wine was a powerful tool for diplomacy. Monastic abbots often would send barrels of wine to curry favor with both lay and clerical authorities.
• The church has a significant land owner during the middle ages and many of their most valuable parcels of land were either vineyards when they received them or were located in already renowned winegrowing areas. It would be natural for them to grow wine.
• People in the middle ages considered wine (or beer when wine wasnt available) a natural part of life so it made sense to produce it in-house when possible.
• Both wine and beer were one of if not the most important trade goods during the middle ages. Monasteries in particular were excellent commercial administrators so they got involved in this trade to increase the financial viability of their institutions.

If you have any questions about any of these points or want any of them fleshed out please do not hesitate to ask.