Same with my parents! It's the only one they use. I think the simple and stable web interface goes a long way; the ChatGPT site (for example) bombards you with popups, new buttons, and opaque daily limits, while DeepSeek's is pretty consistent and straightforward.

DeepSeek also tends to follow prompts more closely IME, plus the thinking is shown, so I think it's able to register as a 'tool' more easily for the non-tech-inclined for whom that appeals.