I have not conducted any scientific research myself! However, Google Ngrams (https://books.google.com/ngrams) show that "solitude" was a much more common word than "loneliness" until 1940. Only around 1970 did "loneliness" finally overtake "solitude" in literature. So some of this can be explained by changing fashions. Ngrams only look at printed text, and I would expect "solitude" to be even less used in spoken language. It's quite a poetic word. In contrast, "loneliness" is almost in fashion at the moment, with many social science and medical studies on the topic, especially related to the elderly. However, this is my own observation, I can provide no data on this.