Explanation: In historical academia, it can take... a considerable amount of time for established views to change. The lack of attention paid to LGBT history and the taboo nature of the subject up until the 1950s meant that very little exploration of the subject was done, and figures we now consider or strongly suspect to be LGBT had such 'troubling' aspects of their lives ignored, glossed over, or denied. Things have changed, though only slowly - as recently as the 90s you can still find respected works which try their best to tiptoe around LGBT subjects.
Nowadays, it's much better, though - LGBT history is a respected-if-still-niche topic, and fewer historians feel the need to deny or ignore the idea that major figures of history were GSMs.