The time-honoured tradition of the Chatan Torah and Chatan Bereshit giving a kiddush on Shabbat Bereshit echoes a custom from the Middle Ages when those two dignitaries would make a feast for the community. (Like most of the Simchat Torah celebrations that we have today, this originated a mere 600-700 years ago.)
Why is the word chatan used to describe the two people called up to say the blessings on the last section of the old year's Torah reading and the first section of the new cycle? Chatan means a bridegroom or a son-in-law. It comes from the verb meaning to tie, connect or covenant.
According to Jastrow's Talmudical Dictionary, this derives from the Assyrian word, chatanu, meaning to protect, obviously a function of bridegrooms in ancient societies.
A chatan becomes tied, connected and covenanted to his new family. So too, the idea of the Chatan Torah may be that the person so honoured becomes deeply connected to the Torah as if married to it.
But chatan has another range of associations as well. It's applied to winning all kinds of distinction; chatan haneshef, for example, means a guest of honour.
Chatan Pras Hanobel means a Nobel Prize winner. The Chatan Torah and Chatan Bereshit are the stars of the day in this sense also.