What makes a pacemaker necessary for atrial fibrillation? My husband is in and out of it every few days, weak, intolerant of beta blockers and has bp.

It depends. Pacemakers might be implanted as part of a care plan for af - but this only prevents slow pulses that might come as a result of slow pulse with af, medications for the af or after av-node ablation. Pacemakers themselves can not prevent or control af. A pacemaker may help your husband tolerate bblockers at higher dose (prevent the slow pulse) and therefore more effectively treat his af.
Definitive Control. In people who have failed other attempts, we can electrically disconnect the top and bottom chambers, controlling the lower chambers with a pacemaker afterward. This provides excellent symptom control in well-selected patients but does nothing to stop the fibrillation. Long term blood thinners are still required.
Pacemaker and AFib. Pacemaker are sometimes placed in people who have atrial fibrillation and need to take meds to stop it, but there heart rate is low. The pacemaker serves to keep the heart rate from going too low, but won't keep someone out of atrial fib. Medicines called antiarrhhythmics and control of blood pressure will keep patients out of af.
Intolerant to BBs. An essential component in controlling af is controlling heart rate which tends to be too fast if not treated. Beta blockers are effective but some people are overly sensitive and have a hr that is too slow. A pacemaker is then indicated to avoid xs bradycardia. Some antiarrhythmic drugs require simultaneous use of a bb for safety as well. A pm protects against pauses and slow hr.