MROs are reluctant to take on work experience students because:
1. You will be non-productive for a good part of the two years
2. They will more than likely have to teach you hand skills
3. You will need to be 100% supervised
Due to these three facts, there is no gain for the MRO in offering you a work placement. In fact, it costs them money because the experienced engineer supervising you will take a lot longer to get the job done therefore productivity goes down and cost goes up. Those are the hard facts of it. They only gain for the MRO is the satisfaction of putting something back into the community by helping trainees.
Can you do anything that removes any of those three? Do you have any experience on aircraft at all? What level of handskills training do you get on your course? Does your training organisation have any relationships with MROs to help you get a placement? Did your training organisation give you a heads up on how difficult it is to get a placement before you signed up with them?
It will not be easy to get a placement but it is possible if you persist with it and do what you can to make yourself attractive to an MRO. As Turin has said below, good luck to you.