Deprecation

I have a rather ignorant, non-programmer’s question for all of you out there: What is the point of deprecating tags? That is, wouldn’t it make sense to be backward-compatible with each decade’s standard? I mean, sometimes I have to access old site information through Archive.org and a lot of the formatting is screwed because the HTML5 specs don’t include those tags anymore, especially those that use the old or tags. I realize it’s up to each browser developer to include/exclude tag support, but I think it would make sense to interpret everything. In the case where both HTML3 and HTML5 format elements conflict, wouldn’t it make sense to set an interpretation order of precedence for newer over the older? Anyway, set me straight folks.