Bitcoin has attracted a millions of new users after the exponential growth in 2017. Which caused bitcoin network to slow down and increased the transaction fees. The network is facing this scaling issue due to the increased number of volumes generated by the new as well as existing users. I believe a lot of exchanges are simple not ready to handle such kind of huge volume and that is the reason why they are on the risk of crashing. That's why they are denying new user registration. However, I believe once the network normalizes, the exchanges who are denying new users will open up for new registration again. I see that Poloniex is still accepting new users now however, you may experience delay in verification completion.

It's not only the new users that causes problems for the exchanges. Most of the biggest exchanges have faced regulations to implement KYC and AML as soon as possible. This means, all of the users have to verify themself with some ID or driving licence and they have to prove their state of residence too.
This is a huge task for the exchanges to handle this verification process for the existing users, and also the new users wants to register, so they just can't handle the workload. They have to meet the AML and KYC requirements, so they just can't accept new users as long as they have problem with the existing ones.

TUXexchange.com

No KYC nonsense..  Check it out..

To be honest i think this whole idea of limiting the number of users on these exchanges is all in an effort to avoid losing control from the large pool of users registering and using these services as bitcoin has attracted lots of people on these platforms...

They received so many new members over loaded the exchange.
Sadly, you have to wait for them to reopen their doors.
Bitfinex stated they will start accepting new accounts Jan15th.

Please do not buy someone else's account.
Binance posted they will ban any accounts caught doing this and there goes your money.

You could get a hot wallet and push some or all of your currencies into there until you can get into a more reliable exchange.

Thats most likely I will do.

There are accounts been sold by 5000 usd....

What are they doing!!??

the exchange site youve mentioned are postpone the registration form for the new users, because they want to upgrade their system to not crush or make more network traffic in the future, so you better to wait until they will announce that they are accepting new users in their site. or you better to move on some other new trusted exchange site.

To avoid the fact that newly created accounts will be used by hackers to steal information from commercial sites, they will not allow new users to register. In part, the number of registrants who have achieved their goals, so that better management of the users is not necessary to register more, but the work of the commercial site is good support for investors.

Nowadays, Crypto currency attract more users as it grows and spread in our community. But not all of these users are can be trusted.
As many people start participating in the crypto world, some of them only joined is just to make fraud as they always do. And most of them are considered all newbies. And also, as a trader, are you going to make transaction to  those people who have no enough knowledge to crypto? Ofcourse not isn't it? Because like we always do. We will choose those people who already have knowledge and related to crypto for a easier trading transaction.