Many good ideas have already been articulated, and some of this will rehash some of them, but:
1. Being highly intelligent and having good judgment are not strictly the same thing. Perhaps this BBEG is prideful (being a drow wizard, I would be highly surprised if they weren't), and thus underestimates what will contain the characters, or (as noted very early) trusts the wrong jailers. Alternatively, perhaps there's some weird skill or unique trick that isn't well known, but which the PCs already have, that makes things work. The big problem here is ensuring that the mistake is one that could be reasonably made--e.g. the jailers might normally be reliable but something has changed recently, or the BBEG has no reason to have carefully researched every party member (or has but some bits are just too difficult to discover/predict: consider The Flash escaping from Justice Lord Batman's prison by making his heart beat faster than the machine could detect.)
2. Drow are cruel and often use surface-dwellers as slaves, sometimes for experimentation. Perhaps an experiment (from this wizard's perspective) hits Gone Horribly Right territory, where something intended to develop new tools for empowering drow ends up instead giving the party the power to escape on their own. Or perhaps the experiment appears to fail, only to manifest later, after the party has gone back to their cells. The big problem here is that you have to give your PCs a new power to do it. Doing so without (a) making them overpowered in the future, or (b) making them feel cheated if it is lost or too situationally-specific to be useful later, may be tricky.
3. Though it's always a risky move...you could have an NPC sail in and rescue them. For a price, naturally. It doesn't have to be extortionary, it could be "I risked a lot to save you, now I need your help to make sure we get out of this alive" or even "we now owe a favor to the Big Good and know it will someday get called in." But it could totally also be a not-so-great person who now thinks they have the PCs under their thumb; this then changes the plot from "how do we escape from this no-win prison scenario" to "how do we secretly throw off this jerk's control?" The problems here depend on the attitude of the NPC. If they're nominally unfriendly/manipulative, why are they only getting involved now, and why is this NPC willing to make such an enemy or take such a risk? If they're nominally friendly, why don't the heroes just pass the quest off to them, and why did they allow things to get so bad before intervening?
4. Even the best prisons are rarely escape-proof, they just usually require out-of-the-box thinking. Perhaps there's a prisoner already present who had a plan, but couldn't pull it off because there weren't enough people to do it, but the party has changed that. Or perhaps there's a specific procedure that opens a very brief window of opportunity on the regular, so the party can spend time secretly practicing for it and then make their escape. The difficulty here is ensuring that the out-of-the-box thinking is something the characters could figure out, but which the BBEG either hasn't considered or doesn't think is a problem. Keep in mind, IRL many procedures that are supposed to be super-secure are often not fully followed because they're tedious or labor-intensive. The Enigma and Lorenz ciphers, used by the Germans in WWII, were both cracked due to overlooking small mistakes in design and failing to rigorously enforce consistent procedures. The Tiltman Break in particular, which was the watershed moment for cracking Lorenz aka Tunny, only happened because a frustrated encryption officer sent a message twice using the exact same settings but shortened the message the second time, allowing the British to crack the code and extract three thousand characters of cipher key. Even if the BBEG and their minions are all top-notch, no one is totally resistant to the power of frustration and boredom, and that can create an opening.
Long story short: You really do have options, but you'll want to weigh which one will have the best potential for enriching the experience, rather than becoming a millstone around your neck.