Answer:
1. Bad command
2. 50% lower budget than the rest of the major powers
3. Russias wast lands
Step-by-step explanation:
There are many reasons why Russia lost that war but I have chosen these 3 as probably the biggest.
The Russian military didn't have good commanders when compared to the Japanese ones. And one of their navy commanders got killed in the Yellow sea. They suffered a decisive defeat later in the war due to bad judgment and lack of ships.
When it came to the army budget and army skill Russia was lacking not only to other major powers but when compared to Japan too.
And lastly, Russia was very large. It is still large but it was even larger back when it was the Russian Empire. All the well-equipped troops and well-trained troops were stationed on the other side of the country. Japan faced an underequipped and defensive army that they defeated with relentless attacks.