Why youtube decided that ? is because it believes that it will ease the load in their ISP because alot of people will be staying home due to lockdown in relation with corona Virus that is causing a major upset to many nations of the world

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WHY (SD)STANDARD DEFINATION?
some critics thinks that Standard defination consumes less data because is 480p compares to (HD)Highdefination wich consumes 1080p of Pics or video that took more data .in some countries data is quite expensive because watching movies wich is an hour and half Cost more on data
Onother Critics thinks that this standard defination will limit number of people watching youtube as all number of people will be at home doing nothing but watching tv and streaming sites like youtube Netflix and the likes.