Why can I not install Keynote on my MacbookPro?

Why there is no install for KeyNote in this model of the MacBook Pro running 10.13 High Sierra ?

Editor's note
MBP 2011 Last supported macOS 10.13.x

Solution 1:

As mentioned at the bottom of https://www.apple.com/keynote/features/

Pages for Mac, Numbers for Mac, and Keynote for Mac are available on the Mac App Store. macOS Catalina or later required

so if you haven't installed Keynote before you may not be able to download it from the App Store.

Alternativly you can use the web version of Keynote on iCloud or try an application like LibreOffice.

Solution 2:

"Why" questions are often very difficult to answer unless the manufacturer publicly states their reasons.

However, in this particular case, the reason is publicly declared - your Mac is designated as 'obsolete' & therefore has no further support.
Obtaining service for your Apple product after an expired warranty
Apple publishes this list, updated periodically, as a list of Macs which still have hardware service support, but software support goes almost hand in hand with this list.

Generally (but not absolutely) any Mac will have full support for its first 5 years then limited support for the next 5. This tends to mean that after 5 years, whilst you can continue running the Mac, software availability will gradually drop behind. Once you can no longer upgrade to the latest macOS, then you probably have 3 years of continued security & OS updates. After that, you're on your own.

A consequence of this is that High Sierra is now more than 3 entire OS revisions old & is completely out of support.
This historical support does appear in recent years to have shortened, but this does happen periodically. Sometimes the underlying technology in a major OS release means that older hardware & OSes can no longer conform to the newer practises.
This happened in two steps this time, first at Mojave & then with a bigger step to Catalina.

I suppose that leaves us with another "why" question, which we can't answer -
"Why can I not get an older version of an app from the App Store?".