This seems to be a default setting on Windows 2016. For Windows 2012 your sslabs score can be capped to B if you use 1024 bit cipher.
You need to add the following entry to your registry.
Access the following registry location:
Vlid key values are decimal: 1024, 2048, 3072 and 4096.
Add the following DWORD-32bit value in decimal:
“ServerMinKeyBitLength”=2048 or 00000800(in hex)
Please remember always test it before using in production as some browser may stop displaying your website if you dont support right ciphers.
More info can be found her