Bitcoin is still widely seen as a speculative asset, so during the early stages of war or crisis, its price tends to fall along with other risk assets, but it may rebound as the situation evolves..
Yes, the simple reason is that bitcoin is still considered a widely speculative asset like the stock market, it is not a safe haven like gold.
When war breaks out, only assets considered safe havens will increase in price and conversely, assets considered risky (speculative) assets will be dumped. So far, only gold and oil are the only assets that have increased in price, the rest are mostly decreasing in price.
Bitcoin is a potential asset but not yet a safe haven so don't be surprised if its value drops during a war.