Because living tissue could not pass safely through the Planar Bridge, Bolas used an army prepared for exactly that limitation. Amonkhet's Eternals were magically preserved dead, armoured in lazotep and commanded by the God-Eternals. They emerged onto Ravnica as the Dreadhorde.
The Elderspell gave the invasion its true purpose. When a planeswalker died, their spark could be drawn into Bolas. The dragon was not merely eliminating possible rivals; he was consuming the quality that allowed them to cross worlds, trying to rebuild the scale of power that planeswalkers had possessed before the Mending.
That mechanism changed the rhythm of the battle. Every delay strengthened Bolas, and every casualty threatened far more than one life. The blue lights of stolen sparks made his progress visible above a city that seemed to be running out of defenders.