Bear in mind, that bitcoin also has a close relationship with the dark market. In fact, maybe it pioneering its usage with Silk Road. In regard to Monero, I would believe that opening up the door on the centralized exchange would rather make them easily observe any activity, well just like any other coin to be exact. But of course, due to the nature of privacy-centric coin, the weight of its users getting observed more is way more likely.
The close relationship with the dark market has changed since 2013, as few people buy Bitcoin to use it to buy things on the Internet, and the majority consider it a long-term investment. Therefore, you see high activity for Bitcoin on central platforms and less on the dark Internet, which has become its currency, Monero.
It is not possible to monitor activities in Monero. It is very difficult, and therefore there is no company capable of providing a difficult tracking service, not to mention any that are expensive in an open blockchain like Bitcoin.
2013 is a long way. Let's cite some reports from Chainalysis.
2023:
Darknet market activity
In the past several years, many darknet markets have adopted Monero to reduce traceability. For instance, White House Market, one of the most active darknet markets before its closure, encouraged its users to shift from Bitcoin to Monero for transactions and eventually transitioned to only accepting Monero. Other darknet markets, such as AlphaBay and Archetyp, followed similar models. However, Bitcoin is still the most commonly used digital currency on darknet markets.
2020:
Finally, we may see more darknet markets accept, or perhaps even mandate the usage of privacy coins like Monero. Monero uses an obfuscated public ledger to make it more difficult to see the senders, receivers, or amounts of cryptocurrency exchanged on transactions. As of now, Empire appears to be the only major darknet market accepting Monero, but that could change in 2020.
So it is clear according to them darknet market activity is still closely tight with Bitcoin. Monero does offer a more robust and private way of transaction, the way they handle transactions is absolutely not on par with Bitcoin. However, the fact Bitcoin still leads the way in terms of market cap also answers why it is still being widely used.
I would love to see if you have some research/report that suggests otherwise.