The mortar and xbow, while not bad cards, have a significant disadvantage compared to other win conditions. They currently have a deploy time of 4 seconds. This is a long time in Clash Royale, especially in double elixir time. With the introduction of new cards since their last nerf, we have plenty of new counters to these cards, such as the Ice Golem, Electro Wizard, Bowler, and the Elite Barbarians. The long deploy time can allow you opponent to nearly generate enough to play an ice golem to counter the siege card. Players have grown far better since this update, countering the siege cards becomes easier as time moves forward and more cards are released.
The xbow is not a bad card, used correctly it can be one of the more devastating cards in the game. The challenge is keeping it alive long enough to actually do tower damage. The mortar on the other hand has more weaknesses. It is easily countered by someone who knows how to counter it. As a mortar player myself, I manage to win more games due to my opponent not understanding the mortar and its dead zone than actually out playing my opponent.
Now Supercell nerfed them because they were not fun to lose against. They didn't want someone to just play a card and have it take down the opponents tower from their side with ease. The royal giant is not fun to lose against and the damage the tower does to him is minimal, so I see this reason less valid now. Lastly, I'm not really upset when I lose to an Xbow or Mortar. They are difficult cards to master and frankly I'm kinda of impressed when people actually win with them (Same with sparky, but that doesn't occur).
The one second reduction should make it easier to hit the tower as it can always be reset after by and electro wizard, zap, or lighting.