It shouldn't be a Democrat/Republican thing, but it is. Democrats just get more votes from populations that have a more difficult time registering (poor folk, college students that are usually away from their home state and may not be able to register in time in their school's state, etc.). Republicans get more votes from old people, who are more likely to vote, because they already have the day off, don't have families to take care of, are less likely to be moving around every year, etc.
Therefore, Republicans are always looking to add more hurdles in registering to vote and actually voting.
As for funding more polling locations, most cities don't have the money or available space. There's a finite number of gymnasiums and/or public space in any given neighborhood, and population density is usually higher. On top of that, I don't really know why voting access in poor urban neighborhoods is not better, but it isn't.