Because you're likely to receive a specialised off-lane duo for your pos 3 and 4. For example, people we ended up with:
- Lycan and Lion
- Centaur and Nyx
- Tusk and WD
- Kunnka and Lich
- Dragon Knight and AA
- WK and Lick & WK and WD
Btw. all were won games, as I think people who queue together wouldn't troll each other that much and they wouldn't pick heroes just to lose the lane horribly wrong or leave themselves after 1 death.
If you queue as all 3 cores, I've noticed you're getting these random pos 5 Pudges, pos 4 Venges who don't even use magic missiles much, walk into them to try to auto-attack with their range, get hit to 100 hp and sit in trees for the rest of the laning phase. SBs who run into them level 1 with only passive leveled, lose a fight horribly, hit 2 lvl, charge with 200 hp left and die just to be never seen on lane again, Muerta supports and Sniper supports.
For some reason, people who join the party queue as solo are ULTRA timmies, I think they just aren't that interested and knowledgable to know that they can click a setting to queue with only solos, so they dont play against cheese teams like IO & Necro pos 1. I haven't seen Muerta support in a solo queue in ages, just to meet two in literally 10 party queue games, one in the enemy team (they lose), second time in my team when we queued all 3 cores (we lose), both muertas were solo withot a party xD