Well because for a teleporter like Nightcrawler shields, no matter how thin, are comparable to having a gate over the top of a pool. If you walk you won't be able to get through, but you can swim under it since the gate doesn't extend under the water. Nightcrawler, by way of moving through this other dimension, is essentially swimming under any defenses Magneto could hope to put up. Even if he makes a skin tight field around him it won't be able to stop Nightcrawler from teleporting a fist into Magneto's head
In this forum we are talking full potential of all combatants. In the comics just cause Magneto is the villian they have other heroes like Cyclops, Havoc and Iceman hold back and fight stupidly. Thats CIS, which doesn't apply here. Theoritically all Kurt has to do is port his fight through Erik's heart/brain