why low cp pokemon are better to prestige??

Just because you get maximum prestige when all of the pokemon you use are half the CP of the opponent pokemon.. something like that, basically more prestige

Technically it is likely they are better by a fraction of a %, if the same mons have the same CP, but different IV.

This would mean that the lower IV has been trained up to a higher level to achieve the same CP...and uses a higher CpM when in combat.

I will find a link...
Read the 'Attack and Defence' section...
https://pokemongo.gamepress.gg/damage-mechanics
The relevant CpM table
https://pokemongo.gamepress.gg/cp-multiplier

In the end, basically IV makes very little difference for a prestige team. Find a mon that will evolve into the CP range you need...and hope for a moveset.
You will save a lot of stardust building a prestige team this way.

EDIT I answered a question you did not ask.
You get more prestige points the lower the CP of your highest prestiger in the team in comparison to the mons you attack. If the CP of the prestiger is half of that of defender you get the max of 1000 prestige.
Note...the CP is based on the highest CP of any in your team.

low cp pokemon gets more prestige

Prestige points are earned according to the formula:

(CPd/CPa)*500

Where CPd is CP of defending mon, and CPa is CP of attacking mon (NOTE: this is CP of the highest attacking mon out of the six the trainer brings into the friendly gym.)

So, by example if CPd is 2000 and CPa is also 2000, then (2000/2000) * 500 = 1 * 500 = 500
But if CPa is 1000, then (2000/1000) * 500 = 2 * 500 = 1000

so yeah.., more prestige.,