With stronger magnets the part of the signal that contains the actual information decays faster in this case. At the same time the total magnetization, most of

With stronger magnets the part of the signal that contains the actual information decays faster in this case. At the same time the total magnetization, most of which is useless to you, decays slower. And you need to wait until it decays to measure again. This explanation is extremely simplified, and the details here depend on the situation. Relaxation is a critical part of MRI and NMR, and there are different mechanisms that contribute to it. The sensitivity of magnetic resonance methods increases with the magnetic field strength to the power of 3/2. But other effects like the mentioned relaxation cause certain practical issues at higher fields. So you don't always get the full benefit of the higher magnetic field, depending on what exactly you are doing.