Because in many cases the states make it difficult to obtain a valid ID to vote. For example, shutting down DMVs or limiting their hours. I'm against voter ID laws until IDs are easily accessible to every voter.
Well it is, but only archaic voting systems would require it and this becomes a nonissue if states shift to modern system like those used in Oregon, Washington, Colorado, and Utah.
Just about everyone I know who leans even slightly left here in the US believes that requiring ID to vote is defacto voter suppression.