My favorite part of the Delver of Secrets era in Standard was casting Ponder. The skill-testing nature of this highly efficient card was what made that deck great in the hands of good players and merely average in the hands of lesser ones. So back in 2011, when Modern was still fairly new, it was kind of heartbreaking to see the two best one-mana cantrips banned out of the format.
These cards are fun because they can be put in any blue deck at an extremely low opportunity cost. I don't hesitate to pick these highly in Cube drafts. As long as you're playing blue, your plan will be strengthened by including these cards.
So why are they banned in Modern? Here is the explanation straight from the source:
A large number of blue-red combination decks kept the field less diverse. One thing that made them so efficient was the cards that would find their combinations. Ponder and Preordain were the most widely used of those cards. Banning these should make those combination decks somewhat less efficient without removing the possibility of playing them.
The blue-red decks alluded to here are, of course, Storm and Splinter Twin variants. These decks have continued to see play despite these bannings, using Serum Visions and Sleight of Hand for selection. These legal cards are decidedly worse than the illegal ones, which just goes to show how powerful this type of effect is.
Since Modern lacks Force of Will, combo decks are harder to answer, even for other blue decks. Making combos less consistent is a noble goal for the health of the format, and it's fair to say that the banning of these cards had the desired effect: Storm and Twin are still playable in the format, even outright winning big events on occasion, but they're not oppressive.
The collateral damage, of course, is that Delver of Secrets decks just haven't quite reached top-tier level in the format. I, for one, would love to play a consistent, competitive Delver deck, but without something better than Serum Visions, that deck just hasn't emerged (not for lack of trying, though!).
It's unfortunate that Delver of Secrets decks are taking splash damage in this situation, but unbanning Preordain and Ponder would mean other nerfs would be necessary for the combo decks of Modern. Storm has already seen Seething Song banned and the DCI could likely find some cards to further weaken the deck without killing it.
But what about Twin variants? What could be cut that wouldn't just ban them out of the format completely? Not all of them play Kiki-Jiki, Mirror Breaker, so that would be of minimal help. Banning Splinter Twin would likely just kill the deck, or at least bring it down a couple tiers. Another option is Deceiver Exarch—making the combo easier to break up since Pestermite dies to Lightning Bolt—but would that do enough? Maybe, but we've seen Village Bell-Ringer in these decks before and splashes are easy in Modern. Besides the already banned selection spells, there just isn't a great way to nerf Twin variants.
After the announcement that there would be no more Modern pro tours, Magic developer Tom LaPille tweeted some additional information:
The takeaway here is that Wizards of the Coast is cognizant of the fact that players have invested money, time, and effort into the format, and often into one particular deck. At this point, don't expect to see decks completely banned out of the format—there's too much risk of upsetting the playerbase. Instead, the bans will be targeted to weaken decks that are too powerful. That's what the bannings of Ponder and Preordain have done for combos in Modern, and much as I would personally love to see these cards available for tempo decks, I'm not counting on it anytime soon.