Say your players cast Zone of Truth and now everyone in this 15' radius is compelled to tell the truth. Sure someone might make a few charisma saving throws but eventually they will fail - if drunk they are often considered intoxicated or suffering the poisoned condition - now they may be under the spell faster and have even more difficulty with bluffing / blowing their cover. If the interrogating person(s) are missing information within this ten minute span they can probably just cast the spell again.
True, no one is forced to speak the truth - they just can't lie. But usually when a someone refuses to give straight answers the players immediately know they are hiding something. In D&D language: each attempt to skip a question requires a bluff &/or deception roll. If interrogation includes detect thoughts, mental escape is near-impossible.
As a Dungeon Master your players will eventually learn everything, at least from anyone in this radius. Unless you have an 8th level Glibness spell, you are hooped. How does one survive this invasion into your hard-developed secrets? The answer: let them. That's right, encourage your players to find out everything.
Everyone is, in truth, unreliable narrators: No knows all the whole truth - and even smart creatures are only sure of what they believe. Most ‘boss’ monsters will give their followers harmful & dangerous misinformation - especially if they learn Zone of Truth style magic is being used on their troops.
Emotional people say weird, cryptic &/or odd things. The NPC knows that they are under magical duress and will soon knowingly betray their friends, leaders and superiors. At the least they will be hunted down for such transgressions. Person(s) interrogated will beg for an end, ask for it to stop, stall with screaming and rage, talk about dangers to their loved ones that will die from backlash, weep uncontrollably and even beg for party help (especially if PCs are lvl 5+ / ‘known heroes’). It won't get anyone out of eventually telling the truth but it certainly adds interesting concerns, ethical dilemmas, plots &/or new directions to to explore.
Not all truth is good news. Forced to talk truth does NOT mean enforced kindness: most will gladly describe the harshest of 'truths' under these circumstances. Examples: how players have observably failed, what they should really fear, who is ugly, stupid or otherwise worthy of scorn / outline anyone's weaknesses, clarify why no one should trust anyone else - and more. Any sinister master-plan oriented ('smart') creatures (hags, devils, morkoths, dragons and more) will use the hard truth to further their agenda.
As a DM, this spell is actually your best friend. Casting of any Zone of Truth spell demands everyone listen very carefully, dote on every word. If non-player characters are under that must-talk sway, the DM can expound on ANYTHING: backstory, narrative leads, plot hooks, history, lore, key geographical features, culturally interesting faux pas - anything the DM feels their non-player would want to say. This need not only serve the game's referee. Should the DM feels generous players could find out about secret doors, describe a long standing boss' weakness, point out surprise ambushes, and more. This is a rare dialogue moment where the DM is on-stage and gains ‘spotlight’.
In short: Zone of Truth allows the DM to give vital information to PCs. This builds investment. Use it as best you can / this is the point of D&D.