It’s because the term Paki, and by extension Pakistani, has become a well earned expression of dislike of them by the general English population. Now Pakis can’t bear being called by their true name, and hide behind the coverall “Asian”.
We see the same phenom over and over again -
a group of people is referred to with a label which at first they approve.
But their culture is such that they become negatively known by the actions and behaviors of a certain subset of that group, be it a minority or a majority of the group is of no consequence - the fact is that they don’t address it and correct it.
The label that they preferred originally becomes a pejorative and they insist on being called something else.
Here, we have the Muslim immigrant invaders taking on the group name of those with a overall positive reputation, “Asians”.
I’m sure the true Asians are soon going to be asked to be referred to as something else, and “Asians” will become a pejorative.