Your BTC pond is smaller than you otherwise suggest. Nowhere close to a majority of BTC users are your anon set.

How many BTC users park their coin on exchanges fully KYC'd? How many Bitcoiners use Join Market? How many Bitcoiners coinjoin every spend? How many users coinjoin at all? This is the tragedy of optional privacy. Most users will use defaults. XMR is also not as ubiquitously available on CEXes like BTC is. It's delisted many places.

If you are using XMR you are most likely privacy conscious in the first place. Why would users have to learn to use Monero? There isn't much to learn. You hit send. Privacy is built into Monero on the protocol level. All things equal there are by far many more ways for users to mess up "privacy" on BTC and shoot yourself in the foot. Not only must you attain "privacy" with BTC, you must maintain it going into the future!

Bitcoin can never be private. It is a public blockchain. The most you can hope for is psuedonymity and obfuscation (a weaker form of privacy vs encryption - everything is still visible).

Unlike Bitcoin, Monero's transaction graph is completely hidden. Amounts and recievers aren't even available to see on Monero's blockchain.

You can use a screwdriver as a hammer if you want, but it is not the ideal tool for the job.