First of all it's a major problem of the Masters, they can't -don't know why- get bigger numbers of spectators watch the race live. Same happened at RB Ring this weekend, the main stand was empty, only the tribune at Turn 1 was filled. It's sad to say, but I think the main problem of the series is that they themselves don't know, how to place it: Is it a national GT series with international drivers and locations inside the ADAC motorsport campaign? Or is it an international series with many German race tracks? I can't even say, if they're one of Germany's main motorsport series, because even people here that are interested in motorsport don't know the series at all, most people here only know F1, then DTM as touring car championship and the VLN/24h race at the ring. As long as they can't provide an answer for that, they'll always have trouble gaining more spectators.
Regarding the venue I sadly can't give any reliable statements. Too less series are visiting the Lausitzring each year (or are on TV when racing there). The track itself is great, but I think they were too motivated with all those tribunes, don't know, if they were ever filled after the Indy Cars raced there (that one race where Zanardi lost his legs). Only the Masters and some smaller series are racing there, the DTM withdraw from there and so there is no real comparison possible between different series. In general I'd say that it's a problem of the series, not of the location.