Large, non-nuclear EMPs mostly use explosives. Covering a large battlefield means you’re essentially bringing a massive, single-use explosive charge to the battlefield, staying uncomfortably close enough to benefit from it, and trying to set it off at exactly the right time, because they’re not reloadable. And your enemy is probably thrilled you’re doing this, because it saves them from hauling their own explosives there. (On that note, why are you sitting on this thing instead of dropping it on the enemy?)
This is in addition to whatever shielding you brought, which is likely bulky and conspicuous. And you’re probably not doing combined arms, because shielding infantry and light vehicles from massive explosions is, it is fair to say, something of an unsolved problem.
But wait, you might be thinking. I know there are non-explosive ways to generate EMPs. Yes, there are, but you need a power source for those, and if you have a really good, portable one of those and a consistent supply of fuel to run it, you probably have better uses for it, like powering a modest laser. Oh, also, you’re 100% sure your shielding works perfectly, right? You’ll find out quick if you don’t.