In my opinion it's a combination of several factors, most have already been mentioned:
- First, "bull run exhaustion" in the case of Bitcoin, see @btc78's post. Sideways BTC stretches in a mid-term bullish move lead often to "retail gambling" with alts. From experience, altcoin seasons are frequent 1) near the start of the bull run, 2) in temporary bearish phases (e.g. those corrections known as "Elliott wave 2 and 4") and 3) near the end of the bull run. I think the first phase already has concluded. Solana for example made already phenomenal profits if we look at the performance since early 2023.
- Second, appearance of new altcoins or altcoin/NFT trends, like @mindrust wrote. This is more difficult to predict, but often if a trend "just catches on", it's difficult to stop it for a while, so if everybody is talking about a trend but it hasn't been around still for a long time, those investing fast can
sometimes (!) get high profits.
But there are more:
People have a tendency to project past price movements into the future, which is one of the few orientation options in the volatile crypto market. So they see for example that coins like SOL or AVAX had much higher ATHs in the past (often 2021-22) but then crashed 90-95% in the last bear market. So there seems to be a world full of opportunities.
Now of course some do their homework and thus know that often altcoins will never reach their old ATHs again, see coins like Namecoin or Dash for example. But in other cases there seem to be reasons for the coins to see a sustainable growth. For example, on SOL and AVAX which I mentioned, are supported by many projects which originally come from Ethereum (e.g. Tether), due to their lower transaction fees. These coins also attract people which aren't only short term gambling but invest more "seriously".
This is also a partial answer to the question:
what are the best altcoins to focus on?
I am personally not very fond of highly centralized coins like SOL, TRX, AVAX, but the "Ethereum killer" market could be a solid option for coins which could outperform Bitcoin.
Generally, there are three groups to focus on:
- "solid coins" like Ethereum, which however may underperform Bitcoin.
- possibly "undervalued" coins which were bigger in the past. Here, as I wrote, one has to look at the fundamentals - are these coins really being used, e,g, as smart contract platforms or "payment coins"? Then it may be an opportunity.
- "new hypecoins". Discovering these is difficult but sometimes possible.
In general however I think in this bull run Bitcoin will have a stronger performance in comparison to altcoins than in some past bull runs. The reason is that the ETFs can attract a completely new class of capital.