Is it legal for your employer to cancel your 401K. Yes, your 401K is simply a benefit provided by an employer to employees. So therefore your employer can cancel the said 401K at anytime. Why did they do this? My thoughts are to get out of the employer match that they probably have set up currently. If you like the plan you currently have you might go to them and ask them to continue the plan without the match because right now employers are trying to save money any way they can. If that would be something you would be interested in I would at least try. If they are not interested or if you don't want to ask you have other options.
Your other options are these. You could open a ROTH IRA possibly through the same company but with a smaller yearly limit and it would be after tax money instead of pretax money but there are benefits to that. Another option would be to open a regular IRA but I believe the Roth has more benefits. You could also open a taxable account at another brokerage but I know that's not what you really want to do. Good Luck.

Sorry I forgot to add this to my answer. The money you have currently in the account should be able to stay in the account. Whatever you do don't remove the money unless you transfer it to an IRA without touching it or there are taxes and penalties that will have to be paid. This is most important. Either leave it in the current 401k or move it to a IRA without taking the cash so you don't create taxes and penalties they are substantial. Good Luck.

If that was the case then why was it ONLY my 401k plan that was cancelled?

There is about 40 employee's at this facility and most the workers are Union
I was the only one that had my 401k plan terminated