Because the key `old_name` does not apply to the sidewalk. If I had tagged the sidewalk as `old_name=Brandywine Way`, that would've meant that the sidewalk itself used to be named "Brandywine Way," which is incorrect. `street:old_name` perfectly encapsulates the situation I am accounting for here, where 𝘁𝗵𝗲 𝘀𝘁𝗿𝗲𝗲𝘁 𝙖𝙨𝙨𝙤𝙘𝙞𝙖𝙩𝙚𝙙 𝘄𝗶𝘁𝗵 𝘁𝗵𝗲 𝘀𝗶𝗱𝗲𝘄𝗮𝗹𝗸 used to be called Brandywine Way. 