Because the Tories have upset everyone internationally, so it isn't really an option. If you've been paying attention the EU has been playing a bunch of jobsworth type games with the UK.
Notice how he will do this in 2025, when the current agreement is up for renewel rather than immediately.
You also have the fact rejoin isn't winding the clock back to 2016, firstly we would loose all of our opt outs, things like the rebate, the euro, etc.. I don't think the reality would actually be popular.
Secondly the UK blocked a number of things like the EU Army (personally I think its a terrible idea, countries that don't spend enough looking to combine to "save" money) so it isn't the same EU.
Lastly see above mentioned jobsworth behaviour, I would not be surprised if the EU demanded the UK to complete all the paperwork of a new joiner and drag the process out as long as possible (it takes ~10 years for most countries).
Far better to put the UK on a stable footing and then ask if EU membership is something the UK still wants. It took the 13 years to get to this point, so its unlikely everything will be fixed during the next government. So why bring something like rejoining up?
The issue is end to end encryption.
The law change requires messaging applications to be able to provide messages between people using their service.
In the 00's, messaging applications would have a secure connection between themselves and person A and anouther secure connection between themselves and Person B.
Person A would encrypt the message, send it to the service, who would decrypt it, open a connection to Person B, encrypt the message and send to Person B.
So if the police got a warrent for communications of Person B (say the police think the person is involved in human trafficking), then the messaging service could provide all messages sent to Person B.
Message services have taken themselves out of the loop, Person A now encrypts the message and sends directly to Person B. So the police appear with a warrent and the message service shrugs its shoulders since it hasno means to get the data.
The law effectively requires messaging services to design the apps/service so they can comply with a warrent.
The issue is less encryption and more the balance between your right to privacy and states right to intrude.
This is why banks aren't upset, they aren't talking about back dooring encryption and bank encryption is between you and the bank so they don't have to do/say anything.