why does pledging hammper the price of the shares
Pledging of shares by individuals or retail investors is done for their personal reasons and has no relation with the share price.
Pledging of shares by a company or its promoters is another matter.
Recently, companies were short of cash and had fewer or no opportunities for raising finances. So, many companies or their promoters pledged their shareholding to banks to get loans for the company. Using pledging as a means of financing means that the company is so cash-strapped for even working capital and that they are finding it so difficult to raise money from the market that they can raise it only by pledging shares. This is a reflection of the companys financial health, its market perception and, these days, of the economy.
Also, in doing this, they surrender their voting rights. This is a risk factor and therefore diminishes a companys valuation. It also makes them vulnerable to a hostile takeover.
Again, as for retail investors, here also, if the companys share price goes down below a certain level, the company will have to make immediate payment in whole or part, or pledge more shares. If the compnay cannot do this, the bank will sell the shares and recover that much money.
This is why usually only 50-60% of share value is given as loan against securities.