Why are Unicode messages limited to 70 characters?

Unicode Standard

Connection Software fully supports sending SMS text messages using the Unicode character set (limited to 70 characters per SMS). All our APIs support the sending of longer messages, which we send to the phone as multiple concatenated SMS messages. Please refer to documentation from The Unicode Consortium and the Unicode FAQ for more details about Unicode. All our APIs support the UTF-8 encoding for Unicode message.

Why are Unicode messages limited to 70 characters?
A GSM SMS can contain a message payload of up to 140 octets (8-bit bytes). The GSM Default Alphabet is a 7-bit alphabet, so it’s possible to pack up to (140*8)/7 = 160 characters in a single SMS. Unicode messages are encoded using UCS-2 which requires 2 octets per character, so it is only possible to pack up to 140/2 = 70 characters into a single SMS.