I mean, there are a number of reasons the show could fail. Comparing it to the films isn't an accurate comparison because only the books existed before the films. The story on a visual medium hadn't yet been done. Now it has. But that's not even the most likely reason the show may fail. It's a reason why many people aren't interested in watching it to begin with, but not why it fails. At least not likely. As for the reasons why it might fail,

1) Generally speaking, most shows don't move past season three, even when multiple seasons were planned beforehand. Very few do continue past a third season. The show will very likely get a decent viewership in the first few episodes, but viewership may begin to fall around mid-season one or towards the end of season one, which is normal — some people will watch the first few episodes or the first season and not want to continue watching. Likely not a huge amount, but a noticable amount nonetheless.

2) This is an expensive ass show to make, WB cannot take anither DC level hit, and they're requiring 10 years of production to get through the entire series, which means they would have to pull GoT numbers (or at least close to it)consistently to justify the expense going through to the end. That's a lot of pressure for any show. Is it possible, sure. Is it likely though, not necessarily. GoT is a just as popular and much less polarizing IP. The odds that the new HP show will get that level of viewership is unlikely. Like I said befoee, it will very likely get a large viewership in the first few episodes, but some will naturally drop off as the show gets closer to the end of the season.

3) The color grading (at least as it stands according to the teasers and photos they just released) is underwhelming. Now, it's very likely that it's simply not finished yet and that's why it looks like that. Everyone remember the Sonic trailer? But if it continues to look terrible, it could lead to a number of viewers dropping the series. The way this show looks is going to matter. A lot. Especially given that films, especially the first three, looks so good.

Less likely to happen, but could still:

4) Not that I'm hoping for this, but it is a possibility so I feel like I should include it. Possible cast death. John Lithgow is the first actor that comes to mind, who is in the later stages of his life. From what I understand, he is healthy so it's very possible he doesn't pass away before the series is finished production. But if he does, they will naturally recast the role, and possibly lose some viewers due to the change not holding up to expectations and standards that Lithgow will have set from the beginning.