Very good question, and welcome to our site!
As the abstract of this book makes clear, Pascal was not a systematic philosopher:
As a philosopher, he was most convinced by the long tradition of scepticism, and so refused – like Kierkegaard – to build a philosophical or theological system.
In other words: the Pensées are not necessarily fragmentary and in the form of aphorisms because he died too early to develop a philosophical system out of these thoughts, they are erratic and to a point exploratory by design, ie. they try different ways to express and shape a thought, very much like Hölderlin will write (probably together with Hegel and Schelling) in The Oldest System Program of German Idealism:
The philosopher must possess just as much aesthetic power as the poet. The people without aesthetic sense are our philosophers of the letter. The philosophy of the spirit is an aesthetic philosophy. One cannot be clever in anything, one cannot even reason cleverly in history — without aesthetic sense. It should now be revealed here what those people who do not understand ideas are actually lacking — and candidly enough admit that everything is obscure to them as soon as one goes beyond charts and indices.
Poetry thereby obtains a higher dignity; it becomes again in the end what it was in the beginning — teacher of the human race; for there is no longer any philosophy, any history; poetic art alone will outlive all the rest of the sciences and arts.
It's not a coincidence that Nietzsche, another existentialist like Kierkegaard, wrote in aphorisms as well.
