Why don the third-party applications that used to work with the Nokia 6081 phone and the Nokia 6080 phone work with the Nokia 6090 phone?

Most of those applications were based on a protocol that was specific to the Nokia 6081 and/or 6080 phones. The Nokia 6090 phone supports a standard protocol : AT commands. AT commands can be sent over the standard RS232 plug (also called DB9 or 9-pin serial connector) for the Nokia 6090 phone. Connection to an external device such as a portable PC is made using an ordinary modem cable (a 9-pin male – 9-pin female serial cable, sometimes called a modem cable extension).