The network effect and different coins/block chains are geared towards a different purpose.
"However, from a general point of view, Bitcoin and Ethereum differ in purpose. While Bitcoin is created as an alternative to regular money and is thus a medium of payment transaction and store of value, Ethereum is developed as a platform which facilitates peer-to-peer contracts and applications via its own currency vehicle. While Bitcoin and Ether are both digital currencies, the primary purpose of Ether is not to establish itself as a payment alternative (unlike Bitcoin) but to facilitate and monetize the working of Ethereum to enable developers to build and run distributed applications (ĐApps)."
Bitcoin Vs Ethereum: Driven by Different Purposes
https://www.investopedia.com/articles/investing/031416/bitcoin-vs-ethereum-driven-different-purposes.asp#ixzz567mNULbr
How is it a good medium of payment? Imagine if the dealer sold you a car for bitcoin at the peak (you bought a 40K car) and paid him 2 bitcoins. now he sold you a 40K card for 16K as of today. That would sound like hyperinflation because that bitcoin in his pocket is worth a fraction of what it was worth a few weeks ago.
No one is going to have confidence in a currency that has massive valuation swings. This is why the USD can be used to conduct any transactions you want.
I still find it curious that all the bitcoin atms spotted around my neighborhood are one way ATMS. USD to bitcoin not the other-way around.