Diamond Weapon attacked Midgar because doing so was in keeping with one of the primary functions of WEAPON.

Consider what Hojo says aboard the Highwind.

Hojo: Weapon... Then it really does exist... I didn't believe in it.

Rufus: What does this mean?

Hojo: ...Weapon. Monsters created by the Planet. It appears when the Planet is
in danger, reducing everything to nothingness. That's what was stated in
Professor Gast's report.
Significant text highlighted for emphasis.

As one can see, Hojo attests that the specific monsters of WEAPON (Diamond Weapon, Ultimate Weapon, and so on) reduce "everything in nothingness" in order to aid the Planet. This information, moreover, was taken from Gast, who would have received enormously accurate information from his wife Ifalna. Given that the Planet was threatened by the Mako Reactors, condensing and refining Spirit Energy into Mako, one can presume that Diamond was attempting to reach Midgar because the city had an enormous populace, which destroyed would return a vast amount of Spirit Energy to the Lifestream.

It also seems possible that the monsters of WEAPON are drawn to what might be called "high-risk" regions, as might be suggested by the actions of Ultimate Weapon. During the Ultimate Weapon sidequest, said monster can be fought at a number of locations: initially the lake/destroyed forest, then it will proceed randomly to Midgar, Fort Condor, Mideel, Gongaga, Mt. Nibel, the Northern Crater, and the Corel Reactor. After Ultimate Weapon's HP is reduced to under 20,000, it will hover between Cosmo Canyon and the Ancient Forest.

From this perspective, all of Ultimate Weapon's targets save the "lake," Mideel, the Northern Crater, and the Cosmo Area have one or more Mako reactors in the immediate vicinity. However, of these four areas, the two where Ultimate Weapon can be fought multiple times (Mideel and the Northern Crater) are both regions where the Lifestream itself emerges from Gaia (which would be similar to reactors, as they apparently tap the Lifestream). Moreover, Sapphire attacks Junon (which has an Underwater Reactor), and Emerald moves about under the sea (where Junon's reactor is located). Ruby, for its part, appears near the Gold Saucer, which undoubtedly needs an enormous amount of Mako energy in order to be maintained. From this view, then, Diamond's assault on Midgar can be seen as an attack on a location where the life of the Planet comes to the surface of Gaia.

As to the matter of Meteor, we're told specifically how the magic functions.

Aeris: ...this must be magic. Just what Sephiroth was saying. The Ultimate
Destructive Magic, Meteor. It finds small drifting planets with its magic. And
then collides with them. This Planet might get wiped out entirely...
Basically, use of the Black Materia could force Meteor and Gaia to collide. This being the case, Meteor's apparent homing in on Midgar is coincidental; it is not being drawn so much toward Midgar as it is being drawn to Gaia.

There is absolutely nothing in the game which suggests that Meteor is drawn to Mako rich regions.