Because Gray has lower defense. He got wtf KO'd by Laxu's little underlings.
He's taken plenty of hard hits and fought quite a few strong goes, and keep in mind he was in an anti-magic zone when it happened, and Bixlow is not exactly weak and hit him with a quad blast.
Remember the bit when he defeated one of the stronger Seven Kin of Purgatory?
How can Wendy boost their stats and how can Lucy summon when they'd all be KO'ed or worst case scenario, if Laxus is out for blood, they'd be dead.
Unless, y'know, he has to deal with strong foes in the way, an ice-wall goes up, Lucy's auto-protect summon happens, etc..
If he just ignores the trio to attack them and turns his back to Erza, Natsu, and Gray, all of whom are able to get hits on him, I don't think that's actually a good plan.
I'll note that Gajeel was able to yank Natsu from out of the way of one of Laxus's attacks, so it's not like it's out of the question for the stronger trio to do something.
We must have read different mangas, coz I remember that Laxus, even without using his dragon slayer magic, mas clearly dominating Natsu and Gajeel.
They were scoring more hits on him than the reverse and send him flying even, he was just able to take handle the damage and keep fighting.
Natsu was still able to flame-kick him into the ground, Gajeel stabbed him a bunch, and they both dragon breathed him.
Also, did you miss the part where Natsu actually dropped him at the end of the fight?
With his speed, Laxus could even take out Natsu as well, which leaves just Erza. And Erza will lose.
And now you're saying it's a one-on-one fight?
Natsu was fighting Laxus solo for a bit before Erza, and then Gajeel, showed up.
You seem to be acting as if "Laxus is faster, that means he doesn't have to worry about a foe at all," and that's just not how it worked in practice. He can travel fast as lightning only got about as many attacks as everyone else, and is vulnerable to attack.
When he actually did try and quickly take out Natsu, it wasn't that easy. Only his big attacks seemed in danger of taking him down, and his big attacks weren't so fast, they had enough charge that people were able to act first.
Even Raven Tail was able to strike at him between his attacks, and except for Ivan their entire team was well sub-Gray.
This team was also able to fight Hades for quite awhile who has more magic than Laxus.
Erza is a foe who he needs to deal with one-on-one, having to defend against her means he won't be able to put as much effort onto the others giving them a chance to attack, amp, summon, etc.. Or on the flip side, if he focuses on the others, it gives Erza plenty of opportunity to attack and deal real damage to him.
In summary, your arguments that Wendy can amp them, Lucy has capricorn and Grey is cool too doesnt really help much when Laxus lightning blitz them and KOs Wendy, Lucy and Grey and sends Natsu flying into the air. Only Erza will be able to react and she'd still get smacked hard.
Wow, Laxus has never done that well, ever. His blitz was one person at a time, he can't hit everyone at once like that, his attacks won't one-shot KO Gray, and when he attacks one person, the others will be able to act and attack him, meaning he'll either have to defend or take the hits.
Laxus is a beast, but you're really underestimating, well, pretty much everyone else. Natsu + Gajeel beat him, he was KOed in the end, and this team is something like two, two and a half times stronger.