This is what RSSC told me about the Tracy Arm excursion and explains why you cannot book it at this time.

"The operation of the tour is dependent upon ice flow inside Tracy Arm Fjord. Last season, we were forced to close down the tour a little early due to the very warm weather in Alaska which created more rapid melting and calving of the glaciers. As the glaciers calf, the icebergs created make navigation inside the fjord much more hazardous for both our ship and the tour vessel.

If the ice flow situation improves over last season, our operator will make the tour available to us. This is a very popular tour option and we all are in hopes that Mother Nature will cooperate so Tracy Arm will remain navigable at least until mid September"