This is because a string of panels in series are limited by the weakest link — if one cell is fully shaded, it blocks electricity flow through it, and therefore through the whole string. Bypass diodes mitigate that to some extent. But with electronics costs still falling, it's now possible to use more smaller inverters to connect the solar array to the grid, each one with its own separate string, or even an individual panel (which is a series string of cells).