This issue crops up on here over and over again. There could be a few possible explanations. Possibility no. 1: VM is hopelessly bad at creating web interfaces between its various departments and its customers (plenty of evidence on here for that). Possibility no. 2: VM plans this failed outcome deliberately. There may be other possibilities too.
Whether 1 or 2 applies, the customer is 'nudged' to contact VM but, rather than just accept a ready-made offer, the customer then has to engage with a VM sales agent and the sales agent is given an opportunity to upsell some more services at the time of renewal.
You could also consider looking around for prices with other providers since VM has failed to provide you with a renewal offer. This will give you some useful comparisons should you decide to get in contact with VM.