3.5 is rate limited free, same as 4o (4o's limits are actually more generous). I think the real reason is much simpler - Claude/Anthropic has basically no awareness in the general public compared to Open AI.
The chatGPT site had over 3B visits last month (#11 in Worldwide Traffic). Gemini and Character AI get a few hundred million but Claude doesn't even register in comparison. [0]
Last they reported, OpenAI said they had 200M weekly active users.[1] Anthropic doesn't have anything approaching that.
[0] https://www.similarweb.com/blog/insights/ai-news/chatgpt-top...
[1] https://www.reuters.com/technology/artificial-intelligence/o...
I basically have to tell most of my coworkers to stop using GPT and switch to Claude for coding - Sonnet 3.5 is the first model that I feel isn't wasting my time.
They also had a very limited roll-out at first. Until somewhat recently Canada and Europe were excluded from the list of places they allowed sign-ups from.