Why is Ethereum actually the only cryptocurrency worth GPU mining?

I don't understand why others ASIC resistant cryptocurrencies such as Vertcoin and Monero aren't worth mining. At all. It's not worth buying a high-end GPU to mine these coins even for those who don't have to pay for electricity bill. But I do. And it's pretty expensive here (~$0.121) .

I'm about to buy (1) R9 280x HIS R9 280X IceQ Boost Clock 3GB GDDR5 PCI-E DLDVI-I/HDMI/2xMini DP + (1) 750W PSU. I'm confident that I will be able to ROI, but it seems too risky because nothing else is worth mining with it besides Ethereum.