Lambivac covers fewer diseases, it's all about risk benefit balance.
Young lambs are probably less likely to suffer from all the diseases covered by heptavac before they go for meat, so they're likely to develop immunity quickly and well to the diseases covered by lambivac, so often in a commercial system it is better value for money to cover the lambs with lambivac.
Some areas of the country may have a lower risk of some of the diseases so some farmers will take the risk to only vaccinate with lambivac.
It has also been previously licensed in goats, so owners of goats (that can be poor at creating a response to clostridial vaccines) will frequently use lambivac.
Hope that explains the use of it.
As advised elsewhere, it would be sensible to find out what neighbours or local smallholders are using to be able to share doses as vaccines only last 10 hours once opened.