The quality of altcoins is improving at the top, especially tron
You are kidding, right? TRON is utter garbage.
If we look at CMC for example, it's not so much that altcoins haven gained on Bitcoin, but more so that the quantity of listed altcoins has increased.
This is the correct answer. There is an endless supply of new useless coins and tokens being created on a daily basis that contribute nothing, achieving nothing, do nothing, but they all contribute to diluting these useless numbers. It's why market cap is such a useless metric. I can create a new token with a supply of 1 billion, sell a single token for $10, and decrease bitcoin's market dominance by 3% in the space of an hour. It means nothing.