Basically, exchanges have lower priority to upgrade their Steem exchange nodes. Two things- trading volume is low for Steem and therefore they don't give a priority to Steem and are performing other maintenance. Or, Steem node upgrading is a difficult for them, they are procrastinating and contacting STINC (who are slow to response).

Hitbtc, Kraken, Bithumb, all these top exchanges have disabled transfers for months some years. Steem has known big problems with exchanges. Exchanges like OKex and Bitfinex do not have Steem pair yet.

Steemit corporation has been negligent about exchanges. Usually, more active foundations from lower ranked coins have listed their coins in many more exchanges.

One of the big listings for coins is that most are ICOs and ERC20 (ethereurm) based token. Most exchanges has infrastructure for ERC20 token, therefore, they can easily list another small coin.

But setting up and maintaining Steem node could be hard both technically and financially. STINC has not actively think about this problem. However, their priority is SMT and they lack people & funding.

EOS with similar difficulty was able to be added in many exchanges. They have bigger funding, more developers and aggressive attitude for listing.