Because the 1992 regulations were drawn up when the meaning of man and woman referred to biological (such an unnecessary term which is why it wasn't spelled out in the regulations) men and women. Because everyone knew (as indeed they still do know today) what a man and a woman are.
As the SC made clear as follows (from https://knowingius.org/p/myths-about-for-women-scotland-v)
The judgment mentions sanitary conveniences or facilities several times.
First, at paras 41-42 discussing the Sex Discrimination Act 1975:
41 Certain exceptions were built into the legislation, some of which were repealed long before the whole Act was superseded by the EA 2010. For example, according to section 6(3) as originally enacted, the prohibition on discrimination under section 6(1) and (2) did not apply to employment '“for the purposes of a private household” or where the number of people employed was not more than five. The exception for small employers was repealed by the Sex Discrimination Act 1986 and the private household exception re-enacted in a much narrower form by section 1(2) of the 1986 Act, limiting it to where objection might reasonably be taken by a person living in the home to physical or social contact with someone of the opposite sex.
42 Section 7 of the SDA 1975 as enacted provided the exception which is reflected in the subsequent legislation, namely that discrimination is not unlawful where sex is a genuine occupational qualification (“GOQ”). The exception does not apply to discrimination in the terms and conditions on which a woman is employed; once a woman has been engaged in the job, there can be no genuine occupational reason for giving her less favourable terms and conditions than her male colleagues. The circumstances in which the defence of GOQ could be relied upon included the following:
(a) Where the essential nature of the job called for a man for reasons of physiology (other than physical strength or stamina), or for reasons of authenticity in dramatic performances: section 7(2)(a);
(b) Where the job needed to be held by a man to preserve decency or privacy because it was likely to involve physical contact or where men would be in a state of undress or using sanitary facilities: section 7(2)(b);
(c) Where the job holder had to live in premises provided by the employer and there were no facilities to accommodate women either to sleep separately or to use sanitary facilities. This was subject to the proviso that the exception applied only if it was not reasonable to expect the employer to provide separate facilities: section 7(2)(c);
(d) The job holder worked in a prison or hospital where all the people present were men and it was reasonable that the job should not be held by a woman: section 7(2)(d).
Second, at paras 50-53 summarising the meaning of sex within the Sex Discrimination Act 1975:
50 What we draw from this consideration of the SDA 1975 are the following points.
51 First, there can be no doubt that Parliament intended that the words “man” and “woman” in the SDA 1975 would refer to biological sex - the trans community of course existed at the time but their recognition and protection did not.
52 Secondly, the legislation recognised and accommodated the reasonable expectations of people that in situations where there was physical contact between people, or where people would be undressing together or living in the same premises, or using sanitary facilitiestogether, considerations of privacy and decency required that separate facilities be permitted for men and women.
53 Thirdly, a range of other exceptions were considered necessary and reasonable, particularly (a) in relation to sport and competitive activity where typical masculine physique would give an unfair advantage and (b) where positive action was needed to ensure that there was a reasonable representation of men and women on the boards of certain bodies.
Third, at paras 222-224 in the context of communal accommodation within the Equality Act 2010:
222 There is a specific exemption for communal accommodation in Schedule 23, paragraph 3 which allows for both sex discrimination and gender reassignment discrimination as follows: “(1) A person does not contravene this Act, so far as relating to sex discrimination or gender reassignment discrimination, only because of anything done in relation to - (a) the admission of persons to communal accommodation; (b) the provision of a benefit, facility or service linked to the accommodation.”
223 Communal accommodation is defined as follows:
“(5) Communal accommodation is residential accommodation which includes dormitories or other shared sleeping accommodation which for reasons of privacy should be used only by persons of the same sex.
“(6) Communal accommodation may include - (a) shared sleeping accommodation for men and for women; (b) ordinary sleeping accommodation; (c) residential accommodation all or part of which should be used only by persons of the same sex because of the nature of the sanitary facilities serving the accommodation.”
224 Here too it is plain that sex has its biological meaning. The Inner House however, held at para 59 that “sex” in this context is defined as including birth sex for those still living in that sex, and “acquired sex” for those in possession of a GRC in the opposite gender. In our judgment, this would undermine the very considerations of privacy and decency between the sexes both in the availability of communal sleeping accommodation and in the use of sanitary facilities which the legislation plainly intended to provide for. If sex has a certificated sex meaning it is difficult to envisage any circumstances in which this gateway could sensibly be met since there would be no rational basis for saying that “for reasons of privacy” any communal accommodation and sanitary facilities should be used by women and trans women with a GRC (so legally female but biologically male) only, but not by trans women without a GRC who may be indistinguishable from those in possession of a GRC (and vice versa). This interpretation would run contrary to the plain intention of these provisions.
The Sex Discrimination Act 1975 governed the regulations in 1992 and therefore man and woman mean biological man and woman.