The indexing operation was largely a read operation, depending on the memory of your server and the size of the table it could have largely been in memory, which would have helped. Also, if that field was in other indexes that might have helped too.. Not sure there..
But the drop column statement, that had to visit all the pages of that table and do a read and a write operation. Very expensive.. And in the process of doing that it generates a lot of data into the transaction log..
CEWII