The biggest reason why Monero cannot and will not be the #1 crypto is because it is developed to be 100% anonymous and untraceable. Lawmakers and regulators will not allow something like that to have widespread mainstream use.
Lawmakers and regulators will just legislate and regulate the on and off ramps and the service providers as they are currently doing with Bitcoin, and have done with cash for years.
Anyone who believes that Monero wouldn't be treated much harsher than Bitcoin by governments is delusional.
For instance: Dell, Overstock et al won't be accepting XMR as a payment method. At best it will be used to purchase illegal stuff.
Actually I prefer to asses the regulatory risk of a crypto currency based on the actual rulings and statements provided by relevant government agencies rather than based on third party comments that are more often than not misleading if not outright wrong. It is fair to say that the US Government has been a leader in this area and relevant rulings from FinCEN can be found here. http://fincen.gov/statutes_regs/guidance/html/FIN-2013-G001.html and http://www.fincen.gov/news_room/rp/rulings/pdf/FIN-2014-R001.pdf. I would urge the reader to carefully read the guidance before reaching conclusions.
This guidance refers to the participants in generic virtual currency arrangements, using the terms "user," "exchanger," and "administrator."6 A user is a person that obtains virtual currency to purchase goods or services.7 An exchanger is a person engaged as a business in the exchange of virtual currency for real currency, funds, or other virtual currency. An administrator is a person engaged as a business in issuing (putting into circulation) a virtual currency, and who has the authority to redeem (to withdraw from circulation) such virtual currency.
The key here is that while both Monero and Darkcoin may be aiming to achieve a similar objective, how this objective is achieved can have profound and very different legal and regulatory implications.
Edit: Assessing this kind of risk is a significant reason why I have chosen Monero over Darkcoin. I may of course be proven wrong, but that is what the free market is all about.