Why does Heidegger say that "regarding the personality of a philosopher, our only interest is that he was born at a certain time, that he worked, and that he died."?
Welcome, Joseph Lutz
The quote comes, as you probably know, from Basic Concepts of Aristotelian Philosophy, p. 4 of the Metcalf & Tanzer translation, Indiana University Press, 2009.
Philosophy contra biography
As always with Heidegger one is inclined to hesitate but in this case I think Heidegger's standpoint is straightforward. He is marginalising the life of Aristotle in order to concentrate on what principally interests H. as a philosopher, namely 'some basic concepts of Aristotelian philosophy, specifically through an engagement with the text of the Aristotelian treatises' (Metcalf & Tanzer; italics in text: 3).
Basic concepts are characterised as 'not all, but some, and so presumably the primary matters with which Aristotelian research is occupied' (ibid.). These matters are to be identified from Metaphysics, Bk 5 or Delta (Δ) which is sometimes called Aristotle's 'philosophical lexicon' : principle (arche), cause, element, nature, necessity, oneness, being, substance, identity, prior and posterior, difference, part, potentiality and a number of others which you can readily check - and may already have done so.
The key point is that in examining these concepts, their interconnections and presuppositions, Heidegger is engaged in the purely philosophical task of penetrating and understanding their specific conceptuality (Metcalf & Tanzer: 4). What do they amount to as concepts? How are they grounded in other, previous concepts - those, for instance, of Plato, Socrates and the Presocratics ?
To these questions, no answers can be found and none should be sought in the life of Aristotle as a person. It makes no matter that he was born in Stagira, that he taught Alexander the Great, that he attended Plato's Academy or that in political ill-favour at the end of his life, with the fate of Socrates in mind, he left Athens lest it should 'sin against philosophy a second time'. Such personalia are the dust of philosophy, of no significance.
That at least is my understanding of Heidegger's anti- or non-biographical standpoint.
M. Heidegger, Basic Concepts of Aristotelian Philosophy, tr.Metcalf & Tanzer, Indiana University Press, 2009.
Aristotle, The Metaphysics, tr. H. C Lawson-Tancred, London: Penguin Books, 1998. ISBN 10: 0140446192 ISBN 13: 9780140446197,
Well, whether Heidegger provided a rationale explicitly for that statement is a matter of historical research, and might not even be an easy task to an expert of Heidegger to prove.
But, if you read a lot of philosophy, it's essentially a statement on an approach to studying philosophy which emphasizes the importance of ideas over historical narrative. Did Descartes fall in love? Most philosophers are more interested in the idea "cogito ergo sum" and it's utility in establishing rational certainty as an empistemological method. Did Willard Quine go to Harvard? Most philosophers are more interested in his naturalized epistemology which advocates psychologism as a basis for understanding metaphysical presupposition.
It's good to know that Descartes was French, was born after Socrates, and preceded Quine in the US, and understand that particular philosophies are born, grow, and are relegated to unread texts, but the biographical tends to be viewed as minutiae when viewed from the lens of the Western Canon.
If you're interested in ideas and have to get through this philosophical reading list, you might take the shortcut and gloss over the biographies.