Firstly, because with limping you cant win blinds and antes by making others fold. Raising gives you two ways to win the pot, either you have the best hand or you get folds. You cant get folds with limping.
Secondly, why to limp?
- If its because you have a hand that you like to see the flop but dont feel comfortable enough to bet on it. What does that
tells to your opponent? That you have a weak hand. Its never good to telegraph the strenght of your hands in poker and playing your cards face up. You can be easily exploited once your opponent smell weakness in you, and understands that if you are not comfortable at least minimum betting your hand preflop, you are certainly not comfortable calling a shove or a big bet with your hand. And everyone understand that limping is a show of weakness.
- Based on limping=weakness, you may limp with a premium holding, or a really nice hand to trap others believing you have a weak hand. But thats not a good strategy. Because you allow more players into the pot, making your premium holding
odds to win plummet and even if you manage to make it heads up on the flop, you cant build easily a big pot from a small preflop pot.
While I am pretty sure there are exceptions to this like limping with a marginal hand and flopping a monster, or limping with a premium when you fear that if you raise everyone will fold and there will be no action, (although thats a recipe for disaster, and for "bad beats" lost-aces-when-7 2-flopped trips). The thing with limping I believe is in the long run you are losing money, or at least not winning the most for the 3 reasons described above.