There are three main reasons:

It sets a precedent, one that could be used by any number of groups with a greater affiliation to another site. For instance, Pagans may choose to bury their cremated fellowship in a site they feel particularly close to, or Trottiscliffe school, itself in danger of being closed, could decide that Coldrum needs a time capsule. Who could then deny them that right?

It is pointless. The media coverage and recording of the contents mean there will never be a point to digging it up. Archaeologists will probably not be interested in something they can click on and view online, or view photos and scans of in a local museum.

It goes against everything English Heritage stand for - to protect and conserve - and therefore undermines their position.