Because Kubo didn't want to show all the powers of a shinigami too soon as you could see as the series progressed they have shown more and more abilities of Shingami's, Quincies, and Hollows. I mean the hollows are the biggest example XD at first the were just big dumb mindless things that tlked and wanted to eat human souls, now look at um they are amplified shinigami's XD
But the reason they didn't at first is because wouldn't it be weird if they seemed to be fling right away and it wouldn't give a good basis for the first fights I mean clear advantages over the hollows at first