Think of it this way: theta tells you how much your trade gains or loses every day (all other factors equal). If it is say $10, it is important to see if the $10 applies to $100 trade (in which case you lose/gain 10% per day) or $1000 trades (in which case it is only 1%).
But if your delta $5, then percentage doesn't really tell you much. Same for gamma and vega.