HotS redistributed all WoL content. The reasons for this were numberous, I will provide but a few.
1. The Terran faction already had a completly extended unit set in the previous campaign. Not featuring most of those units would raise questions as to why they were no longer used. Featuring too many new terran units would raise the questing why they were not in WoL.
2. How many alien creepy textures does one need? Atleast part of the texture set overlaps.
3. HotS got a brand new physics engine. This required the revision of most models to take advantage of new functionality. To prevent the old models standing out when used right next to the existing models and to give HotS a major selling point, they decided to go all out and redo most models.
4. HotS is an expansion. Much like how WarCraft III TFT had all the RoC content, HotS has all the WoL content.
5. Due to conent overlapping, including both mods at once could cause huge compatibility clashes (which version of an Siege Tank to use?) so they dicided it would be cleaner to parallel the content into HotS.
HotS is still a worth while expansion. It adds a large number of new zerg units, boss- units and unit variations. Many map developers will find these of great use in producing a varied and good looking map. We also have many new environments such as swamps and snow which were lacking in WoL.