Traditionally, monoclonal antibodies are produced by hybridomas. There are many issues related to hybridoma produced antibodies. The first and most important one is contamination. For example, mice used in mAb production may carry several viruses. There are cases that the presence of some of the viruses has been detected in the hybridomas. Therefore, there is no guarantee that the antibody produced is virus-free.
The second issue is the consistency and reproducibility. The hybridoma cell lines can die off, lose their antibody genes, or simply not grow anymore when taken out of the frozen storage. Repeating the process to make another hybridoma using the exact same antigen, will result in a cell line secreting a different monoclonal antibody. This means your original antibody is not reproducible and is lost forever.
The third issue is that hybridoma technology is quite time consuming. This is largely due to the time required for the animals to develop the immune responses after the injection of the antigen. The entire process, from the first injection to scale up antibody production, can take somewhere between 3 to 6 months.