There is a lot to unpack here, and in particular, it's always difficult to answer "Why" when it comes to choices made concurrently by thousands of people across hundreds of years. But I can help a little, at least.

First, people didn't really "avoid bathing." Some people did, to be sure. We have records of a number of characters, like Saint Fintan, who is said to have only bathed once a year, before Easter. But this isn't really normal. In fact, the story seems to suggest the opposite, on further inspection: This is a man who doesn't bathe, but he will do so for God.

And we have many, many references to bathing as being essential to good health. It exists in 'Regimen sanitatis,' with numerous rules and pieces of advice for how best to bathe to support your health. Most monasteries had baths, and we have records of paid bath attendants at Westminster. Many Italian towns were known for their famous baths, and people traveled to use them.

There was a decline of the Roman bath culture following the collapse of Rome's famous aqueducts and reservoirs. While some towns, like Bath, England, continued to use their gigantic bathhouse through the middle ages, most of Christian Europe's elite turned bathing into a more private affair. King John traveled with his bathtub; he did not travel to bath houses.

That said, many cities continued to house large bathing complexes: Paris is said to have 32 in the 13th century. It was mostly an issue of resources.

As far as other hygiene, Europeans in the middle ages were fairly normal as well: they tried to clean their teeth, they made sustained attempts at medicine, and they did in fact throw foul-smelling and rotten things out of their homes. Their systems were crude, to be sure, but that's actually not unique to the medieval period. Without an understanding of Germ theory, most of the entire planet struggled with ideas related to the contamination of groundwaters, whether surface or well.

And even with that said, it's not that they didn't try. It's that sewer systems are difficult. London and Paris, for example, long attempted to divert their wastewater out of the city. This took the form of open ditches initially, that flowed out of the city. Later, they covered large parts of them, creating early sewers. But these were just open channels, and many bacteria were happy to sink out of them, into the groundwater below. Without modern Germ theory and modern industrial wealth, it was nearly impossible for pre-modern people to have a proper sanitation system. They tried: they weren't stupid, and even the uneducated knew there was a connection between filth and disease. But it was difficult to correct this.

I'm moving in a lot of directions. Is there anything you'd like to know in specific about medieval health and hygiene? I'm probably better with more focus.