Does she know you really like them? Does she know that you might miss them from a previous relationship? Lots of ways to bring it up without seeming like you're begging or asking or are upset with her. From my perspective - the guys I give them to know they are getting them because I want to give them, and actually like giving them - I consider it a gift - I expect noting in return but never turn down some reciprocation.
Maybe if you approach her subtly and let her know how great it would be for her to surprise you with one someday - a birthday? Christmas? Someone's wedding? And if a discussion actually starts, very nicely let her know that you do not expect one, that you do not consider a BJ as a condition of keeping the relationship alive, that you do not expect her to swallow your cum - but maybe during foreplay just a few minutes of sucking would be awesome.
Here's a thought. I have a cousin who is a few years older than me, who gave me a lot of pointers as to the art of sucking cock. We'd watch porn together. According to her, she loved giving her friends BJs, and I heard from one of her boyfriends that she was really good at it. Then, she was in a car accident, and had some trauma from the collision. One thing that happened to her was she now has a case of TMD - a disorder in her jaw that makes opening her mouth wide or for extended periods of time very painful. Needless to say, according to her, her BJ "career" has been severely cut. In fact, she has to stop sucking a cock after only a minute of so since she is in pain.
Now - she's transitioned to giving handies, and letting her friends cum in different fun ways - but he loves it when her friends point their cocks into her mouth and cum. That way she can suck fr a minute (maybe), play with the cock or watch him jerk off, then still accept the cum and swallow and enjoy it.
OK - this was a long-winded tribute to a cousin who has made a positive change. None of her friends get BJs anymore - they're off the table. But they still get to cum into - or in some cases - actually in her mouth, just not with all the oral in-mouth workup. They and she don't complain, but I can tell - when we watch porn - that she really would like to give what she calls a "real blowjob" - start to finish in her mouth.
Maybe your girlfriend has some medical issue you don't know about? Worth poking around a bit. When you cum - does she ever taste it? Maybe if you scooped some onto your finger and you sucked it off and said how delicious it was - and asked her to follow suit - if she won't, then its just some very personal issue she has with cocks. If she does - then maybe you two can start enjoying sharing your cum (collect it on her boobs, on your belly, in a glass - wherever).
Just my $0.02