That does not make it a consistent card. The cost of the card is by definition inconsistent and most decks aren't going to let your stuff survive for you to even trade in to get a cheap 2 draw.
Sometimes it is really powerful. Sometimes it is a dead card in your hand for the majority of the game. That is definitely not consistent.
If you're running double equality it is much easier to clear your opponent's board and get a huge discount off of it. Plus Control decks often don't have perfect mana utilization throughout the entire game so paying 4 or 5 mana for the cards is often not that big of a deal.
And keep in mind I'm comparing it to Acolyte of Pain, here. People run acolyte because they want two or more cards off of it. That is not always easy to do with Paladin. Especially if you aren't running a copy of Humility. You give me a choice between Acolyte and Solemn Vigil and I'll pick Solemn Vigil every time.