A piano / vocalist duo is a configuration that can work for many, many situations, is easy to pull together and orchestrate.
My guess is, Bob figured he can get by with just 2 performers, and snare gigs that a 4 piece might not get the call for (I see duos all the time in the entry area for receptions, in hotel lounges with a grand piano, etc.).
Talented professionals are keenly aware of the utility of arrangements, and if only 2 parts are called for in whatever they are trying to accomplish, then that's what they will do. It's not about you, for good or bad. It's about what Bob feels suits the gigs he wants to do, is my take.
I would not take it as an offense or insult. Fewer members to pay, and the ability to do gigs a 4-piece jazz ensemble might not be ideal for, and greater ease of scheduling w/ 2 rather than 4 moving parts is reason enough, so don't take it personally. Moreover, being touchy about your personal feelings of being rejected or not up to par don't help present a picture of professionalism. At the basic level, musicians are like any other trade, sometimes you will get a call and sometimes you won't, there might be a lot of reasons for both that are not a real reflection on you. Just like a plumber. Do your job, don't be a prima donna (not that you are) and don't let your ego get dragged into it. The rest takes care of itself.
Finally, make yourself indispensable, if it really bothers you; ask yourself what you can do to make it impossible for people to pass you over.