Question

why should we not say beautiful,beautifuler ,beautifulest .explain

Answer 1

Beautiful is not a sole word on its own, it has a suffix -ful with the word beauty. 'Beautiful' this word is positive degree form a word. For its degree to be comparative, we add -er at the end, when the word itself have a suffix, second suffix for the word is not applicable.

Also Because "beautifuler" sounds horrible. It already has the -ful suffix, so it would be wrong to slap on another one. Plus the word has lots of syllables as it is.
Thus we have to apply the degree at first instead at last, i.e, 'more'
Others:
more pitiful
more abominable
more pitied
more flagellated
Note that every example has a fixed suffix at the and.

Now about the word beautifulest.
As I have mentioned above Beautiful is not a sole word on its own, it has a suffix -ful with the word beauty. 'Beautiful' this word is positive degree form a word. For its degree to be superlative, we add suffix code -est at the end. But the word itself have a suffix, second suffix for the word is not applicable.
Thus we have to apply the degree at first instead at last, i.e, 'most'.

Other examples:
Most ignoring
Most pitiful
Most interesting
Most aqquiring
Note that every example has a fixed suffix at the and. Also, the same rule applies for any word that have fixed suffix at the end.

Answer 2

Answer:

Because the comparative and superlative degree of beautiful is more beautiful and most beautiful