Hard to say - ETH and other altcoins have been growing like crazy as BTC is stuck...
The Alt coins tend to have a little more utility. All you can do with Bitcoin is per to peer money exchanges but it costs too much, though not as much as ETH. There are some alt coins that do peer to peer fast and cheap. Then of course there are alt coins that do a whole lot more with Blockchain tech than peer to peer exchanges.
I think it has to do with supply and demand. BTC supply is limited, and possibly largely hoarded. Most other alt coins do not have such strict supply constraints. So that’s a good thing in one sense and in the short term, but in the long term as awareness and demand grows it drives people into alternatives.
Yes I know BTC can trade in tiny fractions which should help with the supply issue, but that just makes people crazy - Hey I own 0.00286 BTC! IMO they should split but as I understand it they did or are planning to do the opposite - a reverse split. I could be mistaken.
The idea behind BTC and alt coins is brilliant. Blockchain can be used for many things not just directly related to commerce but for all kinds of record keeping, from the county land commissioner to the stock exchange to ridesharing apps. In practice, I am not sure about BTC’s long term utility. Some have said it is a good store for value. Maybe. Has gone up (and down) a lot. But it’s also a huge threat to banks and credit card companies. 3.5% to run a Visa card at the store, or 3 cents to take an Alt-Coin transaction. No brainer for merchants and consumers.