Artillery's point is not to keep up with your main attacking force. Yes, it would be nice if it could, but then you would need a unit that has a movement value of 2, plus the ability to fire and move in the same turn - (actually it would need a movement value of 3, so it could fire, then move two spaces to keep up with your tanks) - that would make for a pretty unbalanced unit.
You can use artillery and tanks in the same turn many times. Stack them together, bombard a city from two squares away with your artillery (this way you don't have to wait another turn moving them right next to the city), then, only after the target has been softened move your tanks in - with their two movement points - if the enemy city has roads next to it, or your moving over grassland or plains, your tanks will still be able to attack after moving one space. (Make sure you leave good defensive units, like Infantry, stacked with your artillery at all times though).
Once the city is taken, the surrounding area comes under your control. The next turn, if there are roads, you can move your artillery three spaces. This may get you in range (with their ability to fire two squares) to fire on the next city the following turn. It may mean there is a turn of waiting between attacks, but the payoff is that you will lose many less units attacking cities if you can be patient enough to keep moving your artillery in. That is what the unit is designed to do - to soften up the enemy to keep your number of casualties lower than the enemies. This means, as a general, you need to determine when and where to deploy your forces, and in what combination, and in what strength.
The artillery's abilities model how it is in real life, If, in the game, they could keep up with your tanks, that would be unrealistic.