Why don't older stock campaigns use new DLC assets?

So, in the ages of the CD version, you had one expansion, TC. You had the option of loading AoK campaigns and scenarios with only AoK data or TC data, enabling training halberdiers in the Joan of Arc campaign, for example.
In the Steam version, if I own some DLC, I can't build siege towers in the Saladin campaign, for example. Why?
See
https://www.reddit.com/r/aoe2/comments/6zzifa/hd_changing_the_dependencies_of_stock_campaign/