I think it would be very difficult to attract PhDs and PhD students, especially at this stage.
Naturally, most professional economists get paid to educate undergraduates and spend large portions of their day doing so. Therefore, they might not find it so relaxing or fun to do the same in their free time.
Therefore, as @JoaoBotelho mentioned, we need interesting questions for PhDs. However, for the most part, only other PhDs are able to ask questions that would interest other PhDs.
This raises the issue, of why don't we have more high level questions, like math or stats stackexchange?
Most questions economics PhDs have tend to be technical. They already have good economic intuition and often either have expert knowledge of their field or have research assistants to do literature reviews. This leaves mathematics and statistics/econometric questions. However, such questions can be answered quicker and often with higher quality on other stack exchange websites than here. I know many PhD students who post question regularly at math stackexchange and its difficult to convince them, that they should rather ask here, especially at this stage. It's not surprising that people would rather ask a professional mathematician about a math question than a professional economist. Furthermore, cross-posting is discouraged.
Increasing the number of PhDs would help this last issue and encourage people to ask more questions here. However, the vicious cirlce is apparent, as the answer to our problem is actually our goal.
A typical solution to such problems is a "big push" approach, where there is a lot of concentrated advertisement effort to get a large number of PhDs to check out the website around the same time. Of course, this is very difficult to pull off. Nevertheless, I do not think we can come out of this vicious circle by slowly and gradually increasing the number of PhDs over a long period of time,