Because, according to the new rule, it's only permitted in proper names
"Eva's Blumenladen" is the proper name of the shop, what is put on the sign above the door.
"Evas Brille" is just Eva's glasses.
"Eva's Blumenladen" is the proper name of the shop, what is put on the sign above the door.
"Evas Brille" is just Eva's glasses.
In the same way, people will wonder why you spelled "Eva's" in "Eva's Blumenladen" wrong, if you spell it that way.
Yes, if enough people start doing a wrong thing, it'll eventually become "right", and I guess in 100 year's we can put apostrophy's where'ever we wa'nt, but currently it still looks odd, and like something that is foreign to German, and imported from English. Because unlike in English, in German this apostrophy doesn't stand for an omitted letter in the genitive singular ending.