Buying a low PE share assumes two things: a) market has not realised that it is undervalued b) market will realise after some time that it is undervalued then the market will value it correctly (at higher price) and I will get out of the stock at a profit.
Could be low PE stocks are going cheap because they have low value to offer, with less scope of growth etc. After all I can not be the first to spot that a stock is having low PE, why others have not bought it? Could be that others know more about it than I do?