Brook pulled out of England's recent Test tour in India in January, citing personal reasons.
The 25-year-old has now revealed the reason was because his grandmother "was ill and didn't have long left".
Brook on Wednesday confirmed he will not be joining up with Delhi Capitals, who paid £380,000 ($486,000) to sign him in December.
"I can confirm that I have made the very difficult decision not to play in the upcoming IPL," he wrote on Instagram.
"I was so excited to be picked by Delhi Capitals and was so looking forward to joining up with everyone.
"Whilst I don't think I should need to share my personal reasons behind this decision, I know there will be many asking why. So I do want to share this.
"I lost my grandmother last month -- she was a rock to me and I spent a huge amount of my childhood in her home; my attitude to life and love for cricket was shaped by her and my late grandfather."
Brook pulled out of England's five-Test tour to India before the squad flew out from their training camp in the United Arab Emirates.
"I made the decision to leave the India Test tour the night before we flew from Abu Dhabi to India because I was told for the first time that my grandmother was ill and didn't have long left," he said.
"Now that she has passed my family & I are grieving and I need to be around them.
"Over the last few years I have learned to prioritise my mental wellbeing and that of my family's, honestly nothing is more important to me than family."
Brook, who has made four centuries in 12 Test appearances for England, played for Sunrisers Hyderabad in the IPL last season.
But the big-hitting star was released by Hyderabad, who paid £1.35 million for him, after a frustrating debut season in the IPL.
Delhi Capitals are due to play their first match of the new season against Punjab Kings on March 23.
