Hello, I wrote an answer on this subject here.

The main problem is our conflagration of the term 'bathing' with the act of 'washing'. The Roman traditions of 'bathing' - the semi-ritualised visit to a public bathhouse with its massages, pools, scraping, saunas etc. definitely declined (and did so anyway on several occasions during the Roman period as fashions and morals changed) but this doesn't mean that people didn't wash.

Roman opinion itself was split on bathing. The Emperor Marcus Aurelius called the tradition disgusting in his Meditations:

Such as bathing appears to thee,—oil, sweat, dirt, filthy water, all things disgusting,

Celcus in his De Medicina warned readers against visiting the bath house when carrying even a small cut or wound, because the filthy water and warm, humid environment was an incredibly virulent breeding ground for gangrene and other infections.

It's against this context that the Church criticised bathing: not only could lots of people hanging around naked and luxuriating be symptomatic of the decadence and moral turpitude which early medieval Christian moralists placed at the heart of the decline of the Roman Empire, but also the bath house was an incredibly real and potent infection vector for disease, in a period where plague had recently been a significant threat. Often this has been misinterpreted that the Church was somehow against cleanliness, which makes far less sense. We know people washed in rivers and lakes - which itself was far more hygienic than going to a bath house - and literary sources at the time tell us how much early medieval people valued grooming and smelling nice.

People also stop bathing of course as the logistical network underpinning the Roman Empire collapses into disarray. Bath houses typically require armies of slaves to do everything from heating the hypercausts to scraping the punters, and without the ability to muster these, the system falls apart. Ready hot water is limited to those few locations with natural hot springs - Aqua Sulis (modern Bath) in Somerset, for example, where a 10th Century bath house complex was indeed constructed. Simply maintaining what are often vast stone buildings with intricate systems of guttering, plumbing and heating itself becomes increasingly difficult without access to the vast material supplies and labour pool available to the Empire.

In short, therefore, bathing bad, washing still good.