why is my Retirement Savings Contribution Credit half of last year?

The maximum credit you can claim phases out as your income increases.

Depending on your adjusted gross income and tax filing status, you can claim the credit for 50%, 20%, or 10% of the first $2,000 you contribute to a qualifying retirement account, with maximum credits of $1,000, $400, or $200 respectively.

Also, 

• If you and/or your spouse took a taxable distribution from your retirement account during the two years prior to the due date for filing your return (including extensions), that distribution reduces the size of the Savers Credit available to you.
• The Savers Credit is a 'non-refundable' tax credit. That means this credit can reduce the tax you owe to zero, but it can't provide you with a tax refund.

For more information check here.