I assume you're asking why people get rid of their decks in general, instead of why they sell them for money as opposed to trading or giving away for free.
Eventually a deck might no longer resonate with a person. He may no longer ascribe to the symbols or themes the deck espouses, or prefer a different pattern of organization (RWS vs. Thoth vs. TdM, etc.). Tarot decks are so multitudinous that a person may eventually just grow tired of one and no longer want it around.
At my height of collecting, I owned about 35 decks. My living situation changed and I gradually adopted a more minimalist lifestyle, so I got rid of decks that I hadn't used and refuced myself to the essentials, the decks I had repeatedly used and whose symbols still resonated with me. I used to be a Neopagan and heavily used the Robin Wood deck as an alternative to the Christian-themed symbols of many traditional decks. When I left that behind, I no longer felt a connection to the deck, so I gave it away to a Wiccan friend who I knew would benefit from it. She still uses it today and I'm happy it found a home.
When I started deep into Tarot, I collected many Native American-themed decks and enjoyed using them. A year into university I worked with our First Nations group on campus, who taught me about the dangers and history of Indigenous spiritual and cultural exploitation that continues to this day. After that, I found that I couldn't use my Native decks anymore as it felt personally improper for me to do so, so I got rid of them.
Possibly the only regret I have is that I no longer have them around to admire their artwork, but as I wouldn't use them for reading and I want to save space, I don't mind that they're gone.