Because they include extra data with hexadecimal representing jpegs, saving a lot of bom-transaction related data in the blockchain, therefore in the nodes.

Nodes have the configuration to limit this extra data, the standard configuration it 80bytes, but there is bug that this kind of transactions are exploiting, that allows them to go way way over any configuration in the node.

That bug was fixed in the Knots node software, and the standard template os configured with very low extra data limit, which is keeping transactions exploiting the bug out of the block. 

If those transactions sent inscriptions within the limit configured in the knots nodes, they would pass normally. 

 Thank you.