My understanding is that the reluctance, by WR to proof for Steel lies in the belief that making barrels for steel shot can result in undesirable handling qualities . Quoting from their blog post last summer:

“It is the barrels that impart so much of that fabulous handling quality into the guns we love and shoot. We should look after them; and that means, for now at least, picking up the ‘phone and ordering a few slabs of bismuth in cartridges of the appropriate case length, swallowing the cost and getting on with enjoying our season.“