One standard rule not just of game design but of any form of entertainment is to aim for an engagement curve which slowly builds up excitement and then relieves the tension at the peak point.
Game designers usually aim for that curve not just in their overall narrative, but also in smaller events, down to the most basic game loops.
When you execute the turn of a unit in a tactical combat game, then the attack is arguably the most exciting part of it. So it makes sense to put this event at the end of this interaction.
Extra Credits has a nice video about the topic of macro-pacing and micro-pacing: Pacing - How Games Keep Things Exciting.
Encourage offensive gameplay
If the player would attack and then move, they would be encouraged to move their units into a safe position after every attack before the enemy can strike back. This would encourage a very defensive gameplay style where the player takes very little risk.
On the other hand, having to move and then attack encourages a more aggressive playstyle. The player moves their units to the position from which they can attack most efficiently and hope they get rid of all threats before the enemy gets their turn.
There are several reasons why you want a more aggressive playstyle.
- Taking risks is usually exciting for the player (if done well).
- It leads to more fast-paced battles
- It is easier for most strategy games to develop a "good enough" AI to play against an aggressive player than it is to play against a defensive player.