I am not expert or anything, but how is possible to have 1639kB block size?
The SegWit changes which were adopted into the system 6 months ago increased the maximum possible block size to nearly 4 megabytes.
And why is not possible for all blocks to be big as this one?
It is "possible".
Miners choose which transactions to include in the block. If there are enough pending SegWit transactions, and they are all structured with a large amount of witness data, and the miner chooses those transactions for their block, then the block can be bigger.
As I can see it is possible, so I am little confused.
Miners get to choose the transactions that they want to include in their block. Since they are motivated by profit, they will generally choose the transactions that will result in the highest revenue regardless of the size of resulting block.