@KDeogharkar kind of summed up the whole underlying premise of It, but maybe I can elaborate a little further.
Essentially, It lives off of the fears of other's; children believe in monsters, and that they "exist" under the veil of darkness. This quote from the novel confirms this:
What does It
really eat, for instance?[...] Certainly we have all been
taught since earliest childhood that what the monster does when it catches you in the deep wood
is eat you. That is perhaps the worst thing we can conceive. But it’s really faith that monsters
live on, isn’t it? I am led irresistibly to this conclusion: food may be life, but the source of
power is faith, not food. And who is more capable of a total act of faith than a child? It, Derry: The Fourth Interlude
As such, naturally, It's main source of food is the contemporary fears of children; It can simply implement the same formula each time to lure It's adolescent victims; take the shape of what the child fears most and when they're least prepared, and bon appetit! However, when it comes to adults, the formula isn't so simple; their thoughts and fears are more complex and It despises this gross rationality. However, this doesn't mean that It won't occasionally feed on adults, and, more often than not, It will manipulate them in order to provide him his younger victims. This quote summarises this nicely:
It had always fed well on children. Many adults could be used without knowing they had
been used, and It had even fed on a few of the older ones over the years—adults had their own
terrors, and their glands could be tapped, opened so that all the chemicals of fear flooded the
body and salted the meat. But their fears were mostly too complex. The fears of children were
simpler and usually more powerful. The fears of children could often be summoned up in a
single face . . . and if bait were needed, why, what child did not love a clown? It, Under the City, 3
It manipulating the Derry adult populus was sort of touched on in the later scenes of the 2017 movie adaptation when, spoilers, Bev's dad goes mental on Beverly, and Mrs. K tries to prevent Eddie from leaving the house and helping Bev.
Now, with this background in mind, you need to understand that much of Pennywise's influence over his victims is due to their faith in the plausibility of what they see. A child is much more likely to accept the materialisation of, say, a werewolf or a bathroom covered in blood rather than an adult. Does this mean that the blood wasn't actually there? Well, it depends on from whose perspective; the children didn't have such a hard time in believing what they saw seeing as they had already encountered It once before, therefore Pennywise's deception had full control over them, and, hence, the blood was real to them. As for Bev's father, however, this wasn't the case.
Instead (in the novel at least), Bev's father rationalises Bev's fright under the guise that a spider scared her instead. This quote shows this:
“Now explain yourself,” he [Bev's father] said, “and make it quick.”[...]
“There was a spider. A big fat black spider. It . . . it crawled out of the drain and I . . . I
guess it crawled back down.”
“Oh!” He smiled a little at her now, as if pleased by this explanation. “Was that it? Damn!
If you’d told me, Beverly, I never would have hit you. All girls are scared of spiders. Sam Hill!
Why didn’t you speak up?” It, Cleaning Up, 2
Obviously, Pennywise could extend his "power" to make Bev's father see the blood too, but what's the point of that? Doing so would only add to Bev's alarm, and prove to augment her weariness; it's better for It to lull his victims into a false sense of security and take them when they're least prepared.