I am just looking at it from a different perspecitve.
The diocese in question, has not yet formed support groups for Catholics who were abused by clergy. Support groups are just one service that a diocese may provide and the diocese in the OP stated that support groups would be formed in the future. There is no information about the other services this diocese provides. Virtually all the dioceses in the US are provided some type of services although not every diocese provides the same services. Maybe the diocese, for example, was funding individual counseling rather than support groups. We don’t know.
There can be many reasons the diocese has failed to form these groups from a need for qualified facilitators to indecision about schedule, location, etc. We don’t have that information so it would be uncharitable to attribute only bad motivations to the diocese personnel.
A group of individuals decided not to wait on the diocese. Good for them! They found a place to meet. Great! Now they want to move locations to a Catholic property and have not found one that would give permission. That’s unfortunate but it’s not a big deal. They have thier group, they have a place to meet. If they were meeting on Church property, they would not be receiving any more “care for the emotional and spiritual needs” than they have now.
The OP has not shared what reason the diocese personnel gave for why they could not meet or what facilities they were requesting. Who formed the group? Is there involvement by a group such as SNAP or VOTF that might be considered problematice to the diocese? Maybe the diocese has already experienced support groups gone wrong. Without proper facilitation, these kinds of meetings can result in bad situations for the members and the Church. And, if the OP suggested, the diocese is planning a diocisan sponsored group, maybe they don’t want to be in the position of having to “exit” the “unofficial” group when the “official” group kicks off.
The bottom line is that the needs of these individuals are being met, although not exactly as they would like them to be right now. We don’t know what happened at the diocisan office so we can’t possibly answer the OP “Why does the church exclude ministerial sex abuse victims from holding support groups on church grounds?” to his satisfaction. In most diocese, the support services are readily identified, so the real answer is “She doesn’t”.