Why do distributors or wholesalers hide their prices and ask for quotation request from the enquirer/potential buyers?

Because depending on who emails them the price point and services on offer will be wildly different.

For instance, if someone from dell or IBM contacts you, you're going to know to charge them a huge amount of money and offer them your top line services and sales reps but if some small startup contacts you, you'll try to work out a sale and offer them something manageable and affordable.

Also, within B2C you will experience huge user abandonment doing something like this but due to nature of B2B and there generally being less options with longer sales cycles, vendors will experience less abandonment in the sales funnel from the lack of fixed prices.

So to summarize, with longer sales cycles, wildly different client budgets and demands, and less defined products and services within BtoB this sales model makes a lot of sense.