Having known multiple trans people and heard them talk about the arguments for and against early disclosure: Fear.

1. They may not be public about their status, and fear exposure to family or coworkers seeing their public profile.

2. They may fear harassment from transphobes. This could range from DM accusations of pedophilia to religious screeds to doxxing to death threats.

3. They may be trying to avoid "chasers." There are some people for whom a trans body (particularly a transfem body) is a fetish, who don't actually care about the person inside. Plenty of transpeople don't appreciate that kind of attention.

4. Fear of rejection. They may believe that nobody will respond if they're open about not being cis.

Also two less fear-related (and less common) possibilities:

5. Ideology. To some people, specifying "transman" or "transwoman" reinforces a social distinction they find invalidating or don't accept. How many profiles have you seen that specify themselves as "cisman" or "ciswoman"? For these people, it's a way of rejecting cisgender normativity.

6. Maybe they just aren't ready to talk about their genitals yet, or have their first conversation be about their surgical plans or history. Not only can get really repetitive having that be the first conversation with every single match, it means they don't get any of the information they're looking for about a potential partner until much later in the process and have to invest a lot of their own time up front. Just like you want the salient information you care about early on, so do they.