It's a design decision. Some people (like me), use the wishlist to keep track of hundreds of games, but wouldn't want to get notified for price drops for all of these games.
Keeping these pages separate also allows for the views to be cleaner. The wishlist page has already got loads of information about the deal: you got game title, time when the deal appeared, time when it ends, platforms, drm, lowest price, old price, discount percentage, discount code, historical low label. The important information about when the game was added to the wishlist and where is synced from (Steam sync, GOG sync etc) is squeezed into the sidebar as there was no space left elsewhere. Now adding all Alert-related parameters would make the wishlist look very bloated.
In the future, we plan to redesign the alerts view and make managing and transfering games between wishlist and alert pages much simpler.