Some good reasons, some not so good.
The women who are "semi-pros" will usually go right to text, these women are smart and sometimes right to the point and will often immediately suggest a PPM arrangement complete with the "gift" that they require. This is almost like dealing with hookers, but usually at a much reduced rate from the hooker world and usually without a clock. The reason these women want to switch from PM to text is to keep from getting kicked of the site for prostitution.
There is a second category of women who want to immediately switch to text and those are the scammers. They don't want a record of them asking for money before meeting, asking for plane tickets, venmo etc etc etc. also to keep from getting kicked off the site.
It's been my experience that if a POT SB wants to switch to text after the very first PM, they are usually up to no good. If they wait two or three PM's before asking to switch, your odds are much better that they are either a pro or a semi pro and will actually have sex with you instead of fucking you. lol
Speaking strictly for myself, I don't mind going quickly to text, but with the caveat that if she won't even exchange a couple of PM's first she will almost certainly prove to have an agenda which will NOT coincide with yours. I get a lot of PM's from women, especially from quite a ways away from me where the very first PM is "hey text me @555-1212" None of those PM's have ever led to a meeting so far for me, and I only respond to them when I am bored.