In the superb page by Gavin Moorhead, in which he describes the "exceptional" Codex Sinaiticus, we have:
Yet, in the 1930s, Skeat and Milne said:Parchment Assessment of the Codex Sinaiticus
The conservation team discovered that, despite being over 1600 years old, the pages of Codex Sinaiticus held at the British Library consisted of a supple, high quality parchment in relatively good condition. This is difficult to put into context as the only other similar surviving 4th/5th Century parchment codices, Codex Alexandrinus and Codex Vaticanus are at this stage unable to be physically compared with Codex Sinaiticus.
 British Library, MS Royal 1. D. V-VIII.
 The Vatican, Bibl. Vat., Vat. gr. 1209.
So, since the British Library is the owner and conservator of Codex Alexandrinus, why not give at least a general, anecdotal, cursory comparison?The Codex Sinaiticus and Codex Alexandrinus:With Seven Illustrations, 1955
Alexandrinus ... the vellum itself now has a:
'limp, dead appearance in marked contrast to the vellum of the Codex Sinaiticus' p. 37
Is the reason that Alexandrinus is so brittle that it is rarely touched?
Or is there a concern that the comparison might raise some difficult questions about the age of Sinaiticus?
What is the reason?