As pps have pointed out, the NHS is subject to procurement law the same as any public sector entity. Because of its own structure (ie having 'commissioning' bodies) it needs to outsource certain contracts - awarding these contracts is subject to procurement rules. Private sector bidders have a good shot of winning due to access to resources which can (at least on paper) give better services and drive down costs. It spans across everything from ambulances, to health visiting, to sexual health services and so on, and there's likely more out there than people realise because the staff don't necessarily change as they are subject to TUPE transfer.
The NHS can't simply give the contract to a public sector bidder despite the fact they may want to, and if they ignore or attempt to circumvent procurement law in order to do that then the private sector bidder that loses out is well within its rights to throw the money it has into suing the NHS and getting a very considerable payout (of public money) through either an out of court settlement or award by the court. Even if the NHS body decided to just restart to process in order to avoid a contract that goes against the relevant principles and legislation and therefore avoid litigation, running a procurement exercise is a bloody expensive task and takes months and months to do, so it's not a decision they would take lightly.
They aren't just handing contracts to the private sector because they want to (often they are very opposed to the concept), it's because the private sector tender was better than the others in the context of responding to the questions they were asked. If the winning offering is terrible it's generally because either the procurement process was designed poorly and did not ask the right questions of the bidders, or the other offerings (which may be good in practice) were terrible on paper. 'Most Economically Advantageous Tender' doesn't simply mean the cheapest. Price and quality are both normally taken into consideration and given more or less weight than other depending on the importance to the procuring body, so they need to get that balance right.
Trying to cut corners in order to award to an NHS body rather than run a process that risks giving a private bidder a contract ends up being more time consuming and expensive than just doing the job right and ensuring you are asking the right questions of the bidders. If it's important that bidders are able to work with other areas of the service, or that they provide a specific standard of care, and the NHS (or any public sector body when it's putting out a contract opportunity) doesn't ask bidders about their ability to do that, then they can't grumble afterwards when the bidder does exactly what it said in its bid and no more.
The procurement rules are in place in order to stop services just being handed out to the friends and family of the decision makers. Unfortunately the appreciation of the importance of procurement law is not always as robust as one would hope in the public sector. As Otherpeoplesteens pointed out, a well advised body won't award a contract improperly - the issue is most don't want to spend the (considerably less substantial than those for litigation or rectification of a poor service) legal fees to make sure that what they're doing is correct and defendable, and then end up either with a poor service, or needing to defend a claim once they're already up the creek.
The opinion of the person using the boots will not be taken into account.
That may be the case sometimes but absolutely not across the board. Evaluation panels need to be made up of people qualified to review the tenders and generally that does include representatives from the various user groups (better still, separate representatives for nurses, doctors, admin etc as appropriate) in addition to other decision makers. The rules are there, it's up to the public sector to follow them.
I'm not for one second saying the process is perfect, but when it's done properly it can be an incredibly valuable tool. Unfortunately it rarely is, and that's an issue within the public sector, not the private companies.