This is exactly why the card is so heavily overplayed.
It has been said a few times now, but I feel redundancy has value here, given how widespread the issue is.
For Reliquary Tower to be worth including instead of a colored source (basic land), it needs to provide value from its secondary effect the significant majority of the games it is played. Value in this case means more than simply preventing you from discarding a card or two - the weakest card in your hand that would have been discarded needs to be relevant to the outcome of the game, and that is simply so rarely the case. In most circumstances, the best seven cards in hand is effectively just as good as all eight or nine, because your ability to play those cards is constrained by more than simply having them.
This also requires you to consistently consistently reach more than seven cards in hand at the end of each of your turns. Something not easy to do for most decks. Even decks that consistently draw multiple cards a turn will often need to deliberately stunt their own development to pull that off. And then we look to the graveyard. Decks that actively play from their graveyard in some fashion are common, and unlikely to be inconvenienced by discarding a card rather than holding it in hand. The only real value Reliquary Tower has here is the niche of protecting unplayed cards from graveyard disruption, provided the other requirements for its use have also been met. Even then, preventing you from discarding in such a deck can easily become a drawback.
Assuming yhe deck actually meets the requirements for Reliquary Tower to be good, we now get to look at what other utility lands it is competing with. The number of colorless land slots is not limitless, and goes down rapidly as the colors in the deck increases,
The ones that will always be included before Reliquary Tower are Ancient Tomb, Strip Mine, and Wasteland. Other high priorities include, if the deck allows them, Kor Haven, Phyrexian Tower, and Yavimaya Hollow. There is a slew of other utility lands with more niche roles, but are universally better than Reliquary Tower in the decks that want them.
It is simply extraordinary rare that Reliquary Tower's niche is better than the other options available, and the deck simply runs out of allotted space for such lands before it should be added.
Just because the card helped that one time you drew 40 cards from Blue Sun's Zenith and didn't immediately win does not make the card good.
Even against Cyclonic Rift, unless it is cast at the end of my turn explicitly to force me to discard, I will rarely end up losing more than one or two things - little more than an inconvenience.