For the reasons about new customer vs existing customer pricing, have a read through past topics, as suggested by @japitts, where it has been discussed many times over.
VM really is not too bothered about retaining customers, especially those on lower tier packages or broadband-only type of deals, for example. VM want to sell you full bundles of services at higher cost per month.
As regards getting a better price ...
In the past, you could ring up VM to renew and usually get a bit of a discount and that was a fairly reliable certainty.
Now VM's discounts/offers seem to be dynamic and ever-changing (even through a single day) so depending when and who you speak to, you may get different offers and deals. The offer you get at one time may not be there if you want to think about it and call back.
Follow the two links here
https://community.virginmedia.com/t5/Managing-Your-Account-Cable/Shocked/m-p/4882213#M169835
for some excellent advice from @Andrew-G, one of the forum VIPs, on renegotiating.
Success (or not) in getting a better price from VM will depend on having done some research on other alternatives available to you, as well as having a serious intention to walk away and move from VM if they do not provide a competitive offer. Some reports on here suggest you might possibly get a call back from VM with a good offer only when you actually put your notice in to leave but that call back is not guaranteed at all.
If you are a broadband-only customer, and you have an acceptable speed available to you from other providers, then you will almost certainly get better pricing as a new customer elsewhere (which kind of comes full circle back to your original point because other providers also offer new customer discounts!).