Greater availability of ETH
There are currently 93 million ETH and 16 million BTC supply in market.
BTC has a finite supply cap of 21 million, ETH is not capped.
Ethereum needs to have a market cap 5.6 times greater than bitcoins in order for the price per coin to be higher.
At current valuations, Ethereum would effectively need a marketcap of above 240 billion dollars for 1 ETH to be worth more than 1 BTC. (This figure is likely to be much higher in future as Bitcoin grows).
Prevalance of high profile Scandals
Ethereum has already split once due to stolen coins and they have recently had a few more hacks where more than $37 million worth of coins were stolen.
This is certainly not the end of the hacks and there are no guarantees that they wont split again. By splitting the coin, the Ethereum ended up rewarding both the hackers and the unqualified people that got hacked.
##Blockchain bloat.
Ethereum is trying to the Swiss army knife of bock-chains the storage specifications for Ethereum are currently growing much faster than Bitcoin, this means that it will reach the reasonable limits of bandwidth and storage space sooner than Bitcoin. This is a classic Hare and the tortoise situation.
Switch to POS.
At some point ETH is possible to switch to POS, if this occurs then it’s growth will inevitably slow because people will no longer be able to mine the coin. Smaller miners = fewer users. I am sure the ETH team are aware of this and that is why they are considering a hybrid PoW PoS system. If they don’t get the scale right then a switch to PoS may lead to organization.
Don’t get me opposite I believe ETH is here to stay but I don’t think its price increase will be anywhere near as exciting it has been in the past 2 years and currently not converted that ETH will be able to beat BTC before the next new fan-dangled technology comes along.