Why Angular 2 or Angular 4 use banana brackets? Is there any specific reason for that?

I know its not related to any coding but they can follow the same syntax as in Angular 1

<input ng-model="hero.name" placeholder="name" type="text">

AngulAR 2 or its Greater Version Syntax till now

 <input [(ngModel)]="hero.name" placeholder="name" type="text">

If they do so, it would be very helpful for people to adapt change in Angular JS ?

Similarly for ng-repeat, they changed to ngFor

<ul ngFor let-item [ngForOf]="items" let-i="index" [ngForTrackBy]="trackByFn">
  <li>...</li>
</ul>

<ul ng-repeat="item in items track by myTrackingFunction(n)">
<li>...</li>
</ul>