Re the "why": It's basically because existing stocks of the movie have beed exhausted. The distributor then has to decide whether it is to their advantage to do another run, bearing in mind that there may be a need to renegotiate some contracts and that the DVD is unlikely to sell as many copies as a "reprint". It may also be that they are waiting for an anniversary/some other excuse to put out a "special edition". I would also imagine that a lot of distributors are holding back on DVD reissues as HD/BluRay launches very soon.
As for "Brewsters", has it ever been on DVD? If not, there are a number of problems with older movies: rights can be difficult to get (or even to find out who to get them from), there may well be a shedload of contracts to deal with, and there is the difficulty of finding a good print/restoration. When you factor in that it's something of a niche market, it may just be not economically viable.