Liquidity and risk. I like monero a lot, but price movement in a larger asset takes more energy for less change.
If Btc is at 10,000$ and you have 100$ in it, if it loses 5% value you lose less than 100 dollars in XMR at 200$ each that loses 5%.
5% loss in Btc is larger in nominal terms, but not in your share.
Pushing an elephant is harder than pushing a rhino.