How come you can trade some stocks after hours, but not all of them?
After hours trading is limited to so called penny stocks etc.
This should answer most of your questions. 
** TSC’s Guide to Extended Trading Hours **
That’s not true at all. After-hours trading exists for all kinds of stocks, not just penny stocks.
I would just be careful about doing it. Liquidity sucks so you need to make sure you use limits to make sure you don’t buy your shares at an inflated price.
I have tended to use after-hours trading in cases where a stock I still believe it gets beaten down for a lousy earnings report. Folks are so desperate to dump it that I usually end up getting it at a discount to the morning open.
In the past if Jim Cramer mentioned a stock I owned it usually got a monstrous bump in the after-hours period. So I sold it to the saps who are desperate to jump on the stock and buy it back after the opening when it came back in a bit. Haven’t watched the show in a while so I am not sure if the ‘Cramer effect’ still holds.