Time, mostly, manpower and the cost to process.

You plant hemp and step back before the sprout hits you in the eye. You have a harvest the same year.

plant a tree and it needs to be looked after - moved from nursery to where it will be for the next 20 years or more. Time for pests to develop or governments change, it might burn, die from drought - they are a big investment. You could die before you get a return from growing trees.

Meanwhile the hemp comes up every year and it is a great tall stem - 3 or 4 metres, loads of raw materials for little investment of time and resources.

By the way, cotton is another poor choice as it requires pesticides, fertiliser and lots of water. Cotton growing causes inland seas to dry up in places closer to the equator than you can grow hemp.

To harvest hemp you tug it out of the ground, (wear gloves) tie it in bundles, put it over your shoulder and carry it to the cart. You don't need to do much to it to get the fibre, and the other parts of the plant are useful too. You can crush the seed, it makes great food for animals you might like to eat, it gives oil when you crush it - that is a good thing.
You can then grow things in the soil you just cleared. Hemp puts the soil in great condition and that is a good thing too.

It is easy to grow and use hemp - it would make a big difference if it wasn't illegal.