Looking at internet forums, YouTube and just the general internet, one can easily see that numerous gamers are hyped up for certain new IP's such as Yooka-Laylee and Splatoon and even sequels such as Uncharted 4. The former ( Yooka and Laylie ) in fact has raised over 1,000,000 British pounds on kick starter while the latter ( Uncharted 4 ) has all it's hype surrounding an E3 trailer which shows it running at 60fps yet the developers so far have only gotten it running at 37fps.

Now the question is why are gamers so hyped up for these games and new IP's in general? So far apart from the developers and the game's PR team putting these games in a positive light ( and at the end of the day it's their job to do so ) and some gameplay footage these games and new IP's have still not delivered enough to obtain the hype they have achieved.

It's not the first time that gamers have been disappointed when a new IP fails to deliver. An excellant recent example is The Order 1886 which not only failed to deliver on the hype but at the end of the day it was a generally perceived as mediecore. Therefore why do gamers still hype up new IP's even though we've already had a taste, numerous times of disappointment when these new IP's fail to deliver? Is it maybe because gamers at the end of the day are hyped up by very easily?

LAST GUARDIAN E3 2015 HYPE

Because sometimes they deliver like bloodborne did

Are you asking why people are hyped for a spiritual successor to Banjo Kazooie? cos... well I don't really know how to explain that without just directing you to any of Rares 3d platform outings on the N64
As for new IPs getting hype? Sunset overdrive was what looked to me like an awesome new IP that people were luke warm for around here at least, I'm not sure some PR companies are just good at getting their new IP out there very well and some do rubbish jobs of it, I wouldn't generalize and say that "all gamers" hype all new ips, that's just not correct.

Though with every Bloodborne there is an Order 1886, Destiny and Watch Dogs.

Because many gamers are very young and have very little experience. They don't know yet that not every game is the second coming of christ. But they'll learn that lesson soon enough, don't worry.

There are also the exclusive titles which get hype solely because of console war reasons.

IMO, nintendo hype is the most hype, and nintendo usually delivers. I mean look at the smash hype. That was totally worth it, and the excitement for each new nintendo direct and E3 is through the roof with most nintendo fans, and the best thing is. They dont dissappoint. Its also alot of fun with hype, it creates a special bond between us fans and lets gamers for once be really happy together

It's not meant to be a thread about why Yooka and Laylee is hyped, but it does go hand in hand with my point as gamers have hyped it up mostly on the merit of what Rare has done ( Practically Nostalgia since Rare hasn't made a 3D platformer in years ). Sunset Override indeed was a game which in retrospecective deserved more hype and you are definitely correct when you said that some PR companies are better at advertising their game. On the whole not all gamers hype all new IP's. That is true, though at the end of the day all of us have hyped at least ounce a new IP and most new IP's still make a large amount of gamers hyped up.

Because gamers are passionate about games. If a developer has a great track record and made games that you love, why wouldn't you be hyped? Sure, it may be disappointing in the end, but if you have a passion for video games then you really should be hyped up.

And if a dev presents new ideas in a new IP that you haven't experienced before and think you'll enjoy them, surely that should reason enough to get hyped about it?

That is incorrect. The Order is a polarized game. Half of the reviews  in Metacritic gave a 70 or above. Which means at least good. Also I'm quite sure it sold more than a million, digital included (but maybe even just physical). Some really loved the game, others found it shallow. One thing that is objectively true is that the game is too short. RaD could at least add a New Game Plus mode with a very hard difficulty. They have been quite naive on that.

Anyway, to answer your question people like new things. And when they get announced they hope they turn out well.