There are 2 big reasons

1.Most of them are attracted to females

A study have found that 73%-81% of male-to-female transgendered “females” are still sexually attracted to females. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4192544/

A survey of roughly 3000 American trans women showed that at least 60% were attracted to women http://transequality.org/PDFs/NTDS_Report.pdf

In this survey trans women respondents 27% answered gay, lesbian, or same-gender-loving, 20% answered bisexual, 19% heterosexual, 16% pansexual, 6% answered asexual, 6% queer, and 6% did not answer http://www.transequality.org/sites/default/files/docs/usts/USTS%20Full%20Report%20-%20FINAL%201.6.17.pdf

2.Moreover, only 25%-30% of transgender people having undergone gender re-assignment have had any kind of gender-confirming surgery, meaning that more than two-thirds of male-to-female transgendered “females” using women’s public facilities will still have their male genitals intact https://archive.is/ZmzsE