Why is the bandwidth for a matched filter twice the normal nyquist theorem one?
One of these difficulties is,that you need a sampling rate that is higher than the double of the highst frequency in the signal. For that reason, signal paths for sampling include an low-pass filter to get rid of the higher spectrum of the signal that is not used. But the problem which come with the low-pass filter is, that they have an infinit spectrum and can not perfectly cut-off frequencies.

Example:
If you would like to sample a signal and get a bandwith of 20kHz, you musst cut-off at 20kHz but you will still get signal components beyond 20kHz. Because of that, your sampling rate would not be > 40kHz, rather than 50kHz or more. It depends on the signal to noise ratio that your application needs.