Because maybe she isn't and has an illness that is bloating her? And the more you deny preg rumours the more they fly. (Personal experience of severe IBS here.)
Maybe she or immediate relatives have a history of pregnancy loss?
Because maybe a baby is one tiny area of her life she does want to keep private?
Because maybe it isn't Liam's and she's not going to tell what's going on? (The people who are most interested in her potential pregnancy will have heard the rumour going round for years so I'm not repeating it here.)