In the first place it's indeed just a Floodgate, but you have the option to special summon it from the GY in a chain with another trap card which gives you a free monster that you can use for a tribute summon.
So it's Floodgate + Free Tribute fodder.
a 2 for 1 card.
In a more competitive environment, Floodgate is a hit or miss. Almost everyone is aware how the 'BACKROWS' need to be wiped first before you attack, or if you can't, find a way to lock them. An advanced user WILL MOST OF THE TIME, take care of the backrows first before summoning anything. Otherwise, Fgate is better than Canadia if your opponent is lacking hard backrow-removal (witch, trunade,etc)
Continuing, Floodgate mostly screws those who uses monster effects as backrow removals, since they need to be summoned first. The same way between Mirror Wall and WoD, they're marginally the same but becomes better in "specific circumstances".
With Synchro summoning being introduced Canadia is better than Floodgate now. Gives you extra levels. If not Synchros summoning you get a card to tribute. Miles better in my opinion. Floodgate is still good though dont get me wrong!
Floodgate's effect is more powerful. But Yugioh is not always about power, Floodgate's activation window is very strict, compared to Canadia which can be done at anytime. Therefore Canadia proves to be more useful in more situations. The extra monster fodder is just cherry on top.