I don't think it would hurt to ask, though I believe BARussell brings up some really plausible situations. I know Matthew Shiner has posted on this forum before about being just a rehearsal or just a performance SM, and I myself have been just a rehearsal SM and just a performance SM.
Speaking for myself, it has all come down to scheduling. Just before I was to go into prep at a Shakespeare company, I got a job offer for a resident ASM position at a ballet company (that paid more than Shakespeare). The ballet job was set to start just after opening of the Shakespeare show. Since I was able to give over 4 weeks notice, the company decided to keep me for rehearsals and hired a replacement at tech. In another instance, I replaced a SM a few days after opening, because it was known in advance that she had to leave for another project. She and the director were friends, so they agreed that she would get them through rehearsals and opening, then I would take over.
Based on what you said regarding their preference to bring back a previous SM, they may want to work with that person, but their schedule only allows for a certain segment of the time. I wouldn't take it personally. While it may seem strange, it isn't unheard of.