Mainly because there is less NPC interactions. You just carry NPCs around so they prevent you from taking damage. hell, for me, they were just what I'd call "meat shields". By the time I've reached the iron mine most of my party members were dead (Jaheira, Khalid, Xzar and Imoen died so I got Minsc, Edwin, Garrick and Gavin(mod)).
I didn't really get attached to NPCs like I did in bg2. But some ppl like BG1 better because it's a bit less linear than BG2.
Also, journal system was sh*t. You never know which quest you've finished etc. And HORRIBLE resolution. I can never bring myself to play BG1 unless I have easy tutu installed.