It s a matter of scheduling. It s hard to assemble certain players at the same time for an entire project. Sometimes it s the artist s call, or the span of time it takes to record a record, and who s available. There is a certain casting call about who s most suited for that style, etc. It s also a question of expertise.
People feel safe about a certain amount of experience.
If you had to go into surgery, you wouldn t want to go to a surgeon who just got out of medical school. Time tells the tale. Knowing who s capable of encompassing a number of different styles, efficiency in the studio, word of mouth. Having a body of work from which to judge over a period of time that has ensued. People are spending huge amounts of money to get their project done.