Couple of things. Reliance is not present throughout the country and does not have to provide services in unprofitable rural areas.
Also, their 4mbps plans are pretty much a joke based on reviews shared by a lot of their users here. Quality of service has deteriorated massively since these plans were launched.
Plus, they do not have the same problems like BSNL for example... pensions, babus, bureaucracy...
BSNL cannot just go ahead and buy stuff they need. They need to come out with a tender and are often forced to buy substandard stuff because it is cheap. Because this is how government works. They cannot risk being accused of partiality or whatever by selecting better quality hardware that might be lot more efficient for more money. Plus, BSNL bosses do not really have any incentive to become a market leader because it only increases workload for them. It's not like they are going to get bonuses based on improved performance or things like that.