From a quick check, their anti-cannabis laws seems to be not really that old as far as culture goes (and might be related to British colonization, also tied at the hip with opium)

Also:

For recreational usage cannabis is not legal. However, cannabis plays a major role in the traditional culture of the island, with the specific Sinhalese or Sanskrit names virapati (“hero-leaved”), capta (“light-hearted”), ananda (“bliss”), trilok kamaya (“desired in three worlds”) and harshini (“the rejoicers”) indicating its various properties, such as inducing euphoria and heightening sexual energy.

That said, the amount here in this specific case (along with related monk-drug headlines) seems excessive* to the point where I wonder if it was a trade/running/money thing.

If it were legal and solely for their own use it would be much better for them to grow and process their own plants (and consume without combustion).

* unless maybe it was actually for way more than 22 monks, I don’t know how long (properly stored) it could last but maybe over many years (and accounting for bulk losses) prohibition could actually make the numbers make sense