If reports are to be believed, then CBI is doing so because Mayawati's close aide and former UP minister Satish Chandra Mishra visited the CBI chief Ranjit Sinha earlier this year.
Earlier, when CBI had registered an FIR in the DA case in the apex court when the court was hearing the Taj Corridor case, but the Supreme Court termed it as 'illegal.' Thus, now the CBI uses Court's this decision as a shield and says that they cannot file a case again.
However, the court never stopped the CBI from carrying out investigations in the DA case independently.
As per DNA report, "Interestingly, when the CBI was preparing the affidavit defending its stand of not filing the case against Mayawati, her close aide Mishra visited Sinha's official residence several times. Mishra has visited the residence at least 12 times between December 2013 to August 2014. This includes seven visits before April including two visits on April 1 and 3. Both the meetings lasted for 10 to 20 minutes. And the visitors' diary maintained at Sinha's residence has no record of the registration numbers of the cars in which Mishra visited."
The CBI, which is now against registering the case had earlier said in an FIR as per DNA that "Mayawati, being a public servant, while functioning as chief minister of UP and as MP.... has acquired huge movable and immovable assets by corrupt or illegal means in her name as well as in the name of her family members, close relatives and associates which are disproportionate to her known sources of income."