I'm surprised nobody answered this... welcome to FT!

Usually airlines don't just have a single "waiting list" but will allocate scarce seats according to a priority. So right now they probably don't want to give you the seat because they think they might be able to sell it at full price. As the flight draws near, they will release some of those seats to less-revenue generating passengers. Ie discount fares, upgraders etc. They don't want to take off with empty seats.

If they said you are on a "priority waitlist" it's probably because you have some status as a FF. So you will get the seat before another upgrader who doesn't have status.

That's just a generic answer. Hopefully someone with specific knowledge of MH will help out.