User blog comment:Xilinoc/Ch. 670 - The Perfect Crimson/@comment-25897574-20160415040047/@comment-24531362-20160415224726

Bankai is definitely a major drain on a Shinigami's reserves. In all of Hitsugaya's fights thus far, his opponents have either been relatively easy to beat (a la Shawlong or Luppi), part of a larger plot Hitsugaya was working to uncover (a la Gin), or part of a long string of expected fights in which energy efficiency was key (a la Harribel). He may have been simply conserving his trump card for a time when there was no other option.