Why did the Buddha emphasis, over and over again, the necessity to keep the 5 basic precepts PURE in order to progress in meditation?

Keep them for a couple of days while meditating and you will know!

Nothing helps to sustain, enhance and support the results and the progress of meditation as much as spotless sila, a mind in peace with its own morality/virtue/conscience.If you like to give it a try, do the following:

1.) each morning, take the 5 precepts
2.) each evening, go through the 5 precepts
3.) see if your mind (conscience says “outch” at one of them…you did not keep it properly)
4.) change your meditation topic to “reflection on you own virtue”. How? While sitting repeat “keeping the precepts” … or … “pure virtue” or any such thought related to the 5 sila…keep repeating this thought until you find your mind starts to rest on this “concept”/”feeling” of your purity

5.) Enjoy the results…

Cmp. AN X, 2