Pop! Comics rolled out in early 2016, to massive creator negativity.
Pop! Comics is the phoenix return of TokyoPop which has probably the worst reputation in the manga industry. We checked the site out when it first opened to beta and took a massive pass on it due to A) the company is still run by Stu Levy and B) the TOS states that it may change its terms of service at any time, “at our sole discretion, without your consent.” So if say, a One Punch Man popular series popped up there and took off, odds are that Pop! would swap its TOS at 3 am and claim ownership. You as the creator would then have to fly to Hong Kong and let an arbitrator decide if that's the case or not. Oh, and Stu Levy is a lawyer, so good luck winning that legal battle. No thanks.
Also readers are only going to use so many apps. Webtoons and Tapas are both so much larger than Pop! that there's no incentive to add the app. After all most of the series there are already at Webtoons and/or Tapas. Pop! is going to have to score some exclusive comics in order to attract readers and it's pretty doubtful given their history if any creators would sign with them.
Financially we're not so sure how strong TokyoPop is. They got a license from Disney to do some manga but they are on our blacklist so we don't buy any of their product and we're certain many other readers feel the same way. We saw some of their print books at a Barnes and Noble and they later all just disappeared off the shelf all at one time so the assumption is that after 60 or 90 days B&N just returned to the distributor for a full refund. Other stuff you see go from 10 to 5 to 2 to no copies so you know they sold out.
We checked Google Play and they report 5k to 10k installs vs. Tapas 500k to 1M. So the readership there is 1/100th of Tapas. That's probably why no series are updating, it's not worth the time to update.