Because most Monero transactions are traceable, according to research by the National University of Singapore:

https://eprint.iacr.org/2017/338.pdf

If anyone can debunk this I’d love to hear more, but generally I think it’s dangerous to rely on claims of secrecy

Thanks for the link. While it’s good to see the paper overstates the problem, it’s also clear that users should not assume that their Monero transactions are untraceable.

I think the summary of both papers (the initial paper and monero's unofficial response) could be that monero's transactions are reasonably untraceable.

Reasonably private should be good enough for most people.

If the goal is more-private-than-bitcoin, yes, it's less traceable than bitcoin. If the goal is bulletproof-from-authorities, not so much.