I think the biggest problem is that it isn't just a few tanks of water, it's a constantly growing amount. It's expensive and impractical to keep building tanks, it's expensive and impractical to filter out the tritium, and it's questionable whether just pumping it out to sea is acceptable (I imagine that if they were able to dump it far into the ocean away from any land it would be more palatable, but that would also be, you guessed it, impractical and expensive).