1) Didymo mostly covers the bottom of the stream. It's called "rock snot" for a reason. It clings to rocks.
2) Most anglers while wading are in constant connection to the rocks mentioned above with the bottom of their wading boots.
3) Some waders have felt soles which are on the bottom of the shoe. The bottom of the shoe is in most contact with the bottom of the river, where the didymo settles and lives.
4) There are pores in felt soles like a sponge. Once the didymo gets into those pores of the felt, they are hard to remove. Much more harder than removing it from the nooks and crannies of boot and wader seems.
5) Felt soles take a long time to dry. Since didymo is dependant upon moisture to live, if those felt soles are not dry when you go fishing again, those didymo cells could leech out into the water you are fishing now and spread to new waters.
6) Do you want to be the angler responsible for introdcuing that didymo to new waters? Or will you take every precaution you can to stop the spread of didymo and other invasives? As sportsmen, outdoorsmen, people who care about the places we fish, I would hope we all would do whatever it takes to stop this stuff, even if it means giving up the felt soles.