In the beginning of the ICO craze there was some uncertainty over which tokens to accept as part of the sale. It turns out that while it is possible to accept many different crypto currencies the easiest one to use is Ethereum (ETH).
The reason for this is because the ETH has programmable contracts for doing an ICO. This allows users to send ETH to the contract and automatically be given tokens for their purchase.
The problem with this is that there is no KYC (Know your client) restrictions and therefore governments don’t like this kind of thing. The way around this is through doing a whitelisted ETH account setup that will allow only wallets that have been approved to be automatically issued tokens and the deposits made from unauthorised wallets automatically returned.
This is the perfect setup and obviously if you accept multiple digital currencies this is very hard to manage. Hence the reason for limiting the token offerings to only accept ETH.
The other reason for doing only ETH vs multiple currencies is the fluctuations around the value of your token in different digital currencies. Imagine that if you had Bitcoins (BTC) and ETH being accepted. How are you going to set the token sale price when the BTC and ETH prices fluctuate all the time.
You could set the price of the tokens in only ETH and then accept the BTC but issuing the tokens automatically is hard as you need to keep the BTC vs ETH flat or have a dynamic pricing done automatically through an exchange.
This becomes very complicated and therefore its a lot cleaner and simpler to just do the ICO in ETH.