"RPI is still calculated by the ONS due to its popularity among those who benefit from higher interest rates, such as landlords"
Far simpler reason. There are inflation linked gilts tied to RPI.
The DMO [debt management office] said it was going to change. The ratings agencies said if that happened it would be treated as a default by the UK government, triggering downgrading and would be treated as a credit event for CDS contracts.
The DMO backed down.
It's the same with BT. They want to default on their debts.