Crystal growth size in anything (metal, minerals, ice, ect.) is all about how long it takes to grow. A crystal that grows very slowly will allow the time for the molecules to order into their lowest energy state, which will be a highly ordered crystal. If they are forced to solidify too fast they are 'frozen' in place before they have the chance to get in the perfect spot.
By slowly evaporating the solution you would have slowly turned it into a super saturated solution. With something to spark the crystal growth (could be a small piece of dmt that has precipitated or an irregularity in the vessel) slowly more and more molecules of dmt will join it and form a crystal. Since it was evaporating very slowly this allows the time for the molecules to order into their lowest energy state.
Similarly if your aim was to grow very large crystals you could do so by having a temperature controlled freezer that would very slowly lower the temp of your solution. The slower the rate of change in temp, the larger the crystal.
As for why not:
In terms of practicality a month or two is probably longer than most people want to wait. Additionally I wouldn't want to be breathing naptha for that long. If you have somewhere out of the way to evap it, great, but it's worth considering that naptha fumes are a carcinogen, and you don't want to be breathing more of it then you have to.
