Why "People reached" is too low on one of my pages?

It may be based on their settings and how many friends they have. The best way to reach the most people is to have good quality content and make sure it is very relevant to your page

Hi Isa Al,

If you build a less engaged audience then your reach will be reduced.

I, like many others, have found that whenever you pay for Likes, you tend to attract a much less engaged audience. This is true even for Facebook Ads Like campaigns. Many of the people you attract will be targeted because they have a high likeliness of Liking every page they visit and many will likely never return to your page or engage in any way except for that one time.

Nothing wrong in promoting your content and optimizing your campaigns for engagement levels, but whenever you specifically target "Likes" in ad campaigns you tend to build an audience of people with relatively low engagement levels, many which have no genuine interest in your page.

For the most part, the only value in running a Like campaign is to reach a level of "social proof" which may enhance a perception of credibility toward your page. The cost is a less engaged audience. For that reason I suggest that you limit the Like campaigns and focus more on creating highly engaging content and build an audience that actively engages with your content (people who comment and share) and is likely to return, over and over.