Given the studies showing higher rates of chronic infections in Alzheimer's patients (like herpes viruses and periodontal disease), why aren't there more trials like this:

https://clinicaltrials.gov/ct2/show/NCT03282916

>Anti-viral therapy in Alzheimer's disease will investigate the efficacy of treating patients with mild Alzheimer's disease with the U.S.A marketed generic anti-viral drug Valtrex (valacyclovir, 500mg oral tablet). Valacyclovir, titrated to 4 grams per day, repurposed to treat Alzheimer's disease, will be compared to matching placebo in the treatment of 130 mild AD patients (65 valacyclovir, 65 placebo) who test positive for herpes simplex virus-1 (HSV1) or herpes simplex virus-2 (HSV2). The study will be a randomized, double-blind, 18-month Phase II proof of concept trial.

I understand beta-amyloid is still the most likely cause of the disease, but given how lackluster the results of targeting beta-amyloid have been, why can't alternative hypotheses be investigated more fully?