can a company do contract with other entity

It does not mean that hp has given an exclusive contract Croma. You do not want to buy through Croma, you can directly contact hp.

It all depends on the appointment terms and agreements.  If a dealer is not an exclusive dealer, nothing prevents him from selling other branded products.  In the case of the branded product, there should be a condition that the vendor should sell only the products of a specific company only.

The question here is not why Croma is selling only hp. The question is why hp is only sold by Croma.

Due to mutual agreements in between them to promote the other's product as reaching to all is easy for two, then one.

It is a matter entirely between croma and hp and others have no right. I do not think that there is any law in the country against that. Any way you can check with experts whether laws against monopoly and restrictive practices can be applied.

Check with the HP first of all and also check do they have any brand connection.

Like Same Logo design or any type of brand similarity that has same clause.

HP cannot sell the model to any other firm.