Because headphone jack is still universally useful compared to either Bluetooth or Lightning, which are still far from it.
Because some 32 bit apps are not updated, will never be but are still quite useful.
Neither holds Apple back in any way, nor affects anyone who wants to go all-in with Bluetooth, Lightning or only 64 bit apps. Such stuff would simply make Apple products more useful for those that desire such utility.
Take very simple, very common use-cases. For example, ever fly? Ever want to watch the seatback live video instead of whatever you might have on the iDevice? How many mainstream airlines have a Bluetooth connect option for the audio? How many have a Lightning jack for the audio?
Hot girl/guy sits next to you. You strike up a conversation. She/he wants to share a great song on their phone with you. It's not an Apple phone. Did you bring the adapter? If not, tough luck. Or instead of listening together, it turns into a your turn: my turn. How about watching a movie she has on her device together? Tough luck.
Visit a client. They need you to jack into their audio equipment to hear the latest ad/video/conference/broadcast/etc. They're NOT an Apple shop. Offend the client by ridiculing their lack of Apple equipment? Delay the conference so that maybe their IT guy can find some way to jack you into their equipment? Just miss out and look like a disconnected consultant/partner?
This stuff is everywhere. Can't even work wired between iPhone and Macs- must have the adapter or 2 (TWO!) sets of headphone.
It's not "the future" per se, it's still just an ongoing usage hassle. Carry the adapter everywhere and/or be sure this other thing is charged too. Just roll with what you can't do because of the rampant incompatibility outside of a narrow Apple bubble?
"But, but, but the other guys are dropping the headphone jack too." Yes, but they are NOT adopting Lightning. And Bluetooth still has so many limitations in sharing, in quality, etc.
On the app issue, useful apps are useful apps. Sure it would be great if every app was upgraded to be 64bit but neither Apple nor us consumers have any control of that. And if the developer is closed or just doesn't care anymore, that doesn't make their app any less useful to those who use those apps. And there's not always substitutes for every app.
I realize that some of us seem compelled to go with a "whatever Apple thinks/does is the one and only right way for ALL" stance, but I, for one, still subscribe to an earlier Apple decree: "think different" which sometimes applies pretty well when Apple's thinking is working against a consumer's utility. We should be able to think objectively and share our opinions and not necessarily be wrong just because they don't sync with what a corporation has decided... even THAT corporation.
In matters such as the headphone jack and 32-bit app, NOBODY is negatively affected by keeping either- not Apple, not other consumers who only want "the future", etc. However, those who want "the past" can be greatly affected by trying to do what so many do- just roll with it, because we want the whole bad enough to try to roll with the choices that affect us negatively. Some spin that as a unit sale vote in support of Apple's choices but I think that's really a vote to just tolerate the headaches of some Apple choices because we want "the rest" bad enough to do so.
I'm quite the Apple consumer myself. I still HATE the headphone jack decision. It's a terrible fragmentation (driven only by a choice, not being forced there (no "thinner" is not a force)) that puts a burden on us consumers for at least many years to come. There is NO good way to "roll with it" and have any "the future" option work as well. Embrace Bluetooth and reject being able to use the buds with you in many common situations. Embrace lightning and not even be able to use your best headphones with Apple's own Macs. Carrying adapters is a poor substitute to just leaving a tiny hole and tiny channel in for those "old fogeys" who want to keep using the most ubiquitous standard port for audio in the world. Carrying more than one set of headphones to cover all bases is a poor substitute for 30 seconds of "thinner & lighter" spin at the big reveal.
How long until either Lightning or Bluetooth is as available on everything? Never for Lightning and it seems like it will be a very long time for Bluetooth. But hey, let's celebrate our difficulties because a corporation knows best for ALL of us. And let's ridicule our fellow consumers who don't just tow the company line because a concept like "think different" only applies when it aligns with whatever the corporation wants us to buy.