It's not that unusual. They aren't fully out of stock. They come and they go and the places with more need will get more phones. Billy Bob Town with 1 cell tower will not get 10 phones every week, they'll get 1..or 2 at a time and only if they are replacing sold stock. New york stores may get far more for instance.
Keep in mind that you're not the only one seeking out this phone. If online gets 150 in stock, that's an entire supply available to the entire country. It's in demand and as soon as stock hits online they sell very quickly.
As for your local stores. Call them everyday and ask if they got the Evo in. Eventually they will.
This is not the only product that I've seen long term shortages. I lived in Charlotte, NC for a number of years and not so long ago they went weeks of gas stations out of gas. As soon as 1 station got any they pummeled the supply and it was out before the next station got any. Eventually supply rose and all went back to normal.
Now the Evo isn't like gas but enough people want it and supplies are thin enough it appears that it's almost always out of stock.
On a related note I read recently that HTC was having supply issues with their manufacturers of screens for certain models. This could explain (even with other parts) why they are not ramping up supply to meet the demand.