Because whether or not we like it, the law is non-obvious to people who haven't studied it deeply, this is a particularly non-obvious area of the law, and you will be better off with an expert.
Same reason you want to find someone who knows C very well to review your security-sensitive C code. It would be nice if C didn't come with rules about undefined behavior that are surprising to people who haven't deeply studied C, but it does, and no amount of wishing it didn't will change the fact that you are inviting immense and needless risk by not finding an expert (or, if you'd prefer, spending years becoming one yourself, which you can certainly do if you have the luxury of years).
People need attorneys for DUIs, and often for less serious moving violations. So for NSLs, having an attorney is essential. There are two primary reasons. First, you want to comply only as legally required. And second, you want to avoid making mistakes that have criminal penalties.
Edit: Even if your firm has general counsel, it's not uncommon to hire outside attorneys for specific matters.
There's some confusion here about what "you" means. I can't imagine how an individual developer at some firm would receive an NSL individually. So it's the firm that would be deciding whether or not to hire outside counsel.
Because the government has at its disposal many legal tools of which an individual developer may not be properly equipped or knowledgeable enough to deal with.
