Prior to approximately 2004, referrals to gender identity clinics had (for ease of conversation) male patients outnumber female patients by the same number as the reverse, now. It was never seen as an issue that needed to be looked into, then, nor do you even seem to be aware of it, as you gave the claim that the numbers have roughly been even.
Why is it suddenly a problem when the numbers are reversed?
This is incorrect. In children who present with gender dysphoria in early childhood (this includes homosexual transsexuals aka early-onset transsexuals), empirical evidence from 50 years of research shows that the sex ratio was equal (to be precise, it was something like 1.06 male children for every female child).
Male children were referred much earlier than female children, because society tolerates tomboys better than feminine boys, but by around nine or ten years of age enough girls were referred to balance the ratio. That's because the older girls get, the less acceptable it is for them to be gender atypical. By age 18, both sexes are evenly represented amongst early-onset homosexual transsexuals.
There has never been even a 2 : 1 skew towards males amongst early-onset transsexuals, let alone a 7 : 1 one.
Those with adolescent and post adolescent teenage onset are a new cohort. Their sex ratio is heavily weighted towards the female sex, and they present quite differently from early-onset transsexuals (in, for instance, having been gender typical prior to presenting with gender dysphoria, unlike early-onset transsexuals who are extremely gender atypical from a yoing age).
Are you maybe thinking of late-onset transsexuals when you say the ratio has always been in favour of males? Amongst transsexuals in general that is certainly true, due to the fact that there is no equivalent cohort of late-onset female transsexuals, and because late onset transsexuals outnumber early onset ones by quite some measure, but this thread is specifically about children. And for children diagnosed with gender dysphoria this does represent a significant change in the distribution of the sexes.
(I recommend reading Kenneth Zucker and other researchers who primarily focus on children diagnosed with gender dysphoria for some insight into this cohort.)