Most major professors that I know working in the area of ecology offer their graduate students assistantships because
they ask them to do a lot of work. Assistantships are limited because most of us have to write grants to get them and it is difficult to get them funded. If we provide an assistantship to a PhD candidate that is usually a 5 year commitment
and if we do not know the student well we may not want to take the chance of trying to support someone that long - not all graduate students and major professors work out together. So most often they will rather provide an MS
assistantship and if the student works well they will offer a PhD. Also if you get an MS in the US and do a good job it is usually easier to go on for a PhD because you already have proven yourself and have a major professor who will provide a good reference for you. If you were lucky enough to find a PhD assistantship you might find that you do not like the
project and then you are stuck with it for a long time.