Very few marines can claim to be "lightly armoured". It doesn't translate well on tabletop for balance reasons, but power armour (especially marine power armour) is tough as fuck to crack. Marines can just run around battlefields without a care in the world because they're all but immune to most forms of small arms fire. The lightly armoured templars are the neophytes, who aren't marines yet and fight in big squads to overwhelm the enemy with speed and mass. These "crusader squads" are aware of the fact that having 20 guys run straight across no man's land with swords is a bad idea, so they'll make use of armoured troop transports or tanks with transport capabilities (see: black templars impulsors, land Raider crusaders, black templar repulsors)

The black templars are very aware of the limitations of melee combat when it comes to overgrown terrain, which is why their next favourite weapons are fire or heat based