The big reason is consistency. You could argue that Maverick is just a more consistent Knight of the Reliquary deck. Sure, you have better matchups with Chalice of the Void and the ability to play Chalice for 1 on turn 1, but by playing cards like Mox Diamond and Chalice you are losing some consistencies. These cards are great in opening 7s but aren’t as powerful later in the game.
You also have Planeswalkers in 4C Loam that you can rely on as your threat of choice but I’d argue it’s easier to take 4C loam off their threat than it is against a deck like Maverick. 4C Loam’s manabase is also quite unstable and without the Life from the Loam engine, can really suffer under pressure from mana denial effects like Wasteland and Rishadan Port. Maverick on the other hand has a much more stable manabase and access to mana-producing creatures to get you ahead of your opponent.
Chalice and Loam aren’t always the great lock pieces they are known to be in a certain meta either, which can really hurt the overall efficiency and power of 4C Loam. However Maverick’s consistency tends to favour its ability to win matches in the early, mid and late game.