A reason (the only reason?) to decline would be that you do not agree to the terms.
In order to have the credit card in the first place, you have a contract with the bank that spells out the specific terms of your arrangement, which includes defining the interest rate. Changes to this contract require agreement by all parties involved (it's possible the contract could specify that the bank is allowed to change the interest rate at any time, but then changing the rate would not be a change to the contract).
It's possible (although not a given), that this offer has some strings attached in the fine print that benefit the bank. This would make sense; the bank is giving you something of value (lower interest rate), and would expect to get something of value in return (new/higher fees, shorter/no grace period before charging interest, changes to any rewards, permission to sell your information to advertising partners, etc.). Make sure to read the fine print to see what exactly you have to agree to in exchange for the lower rate. If (like many users of this site will strongly advise) you pay off your balance before you ever accrue any interest, the lower rate may not be worth it.