Bitcoin is getting cleaner, while altcoins are getting dirtier.

Just look at the biggest DarkNet marketplaces (the sites that raised
public awareness for Bitcoin back in the day). Most have abandoned Bitcoin due to a
lack of anonymity and several shutdowns of marketplaces due to blockchain
analysis. Instead they are using altcoins like Monero or Zcash, which have lower
fees and offer more anonymity.

This may change in the future with possible enhancements of the privacy features
of BTC (e.g. stuff like MimbleNimble).