The WH said it's due to the site becoming very popular with Americans and wants to make sure they give each petition the full attention it deserves so they are quadrupling the number of signatures required to 100K before the WH will read it.
Note: when the site was created only 5K signatures were needed.
White House now requires ‘We the People’ petitions to have 100,000 signatures for official response | The Daily Caller
“That ‘good problem’ is only getting better, so we’re making another adjustment to ensure we’re able to continue to give the most popular ideas the time they deserve. …