I assume you're talking about OTA (Over The Air) broadcast, which is the "ATSC" (Advanced Television Standards Committee) standard. Each broadcast signal is assigned a maximum of so many bits that can fit within a standard 6 MHz channel.
The more bits you can stuff into a channel, the more information you have, and the higher the picture quality (PQ). Take a look at Table 4 in The ATSC standard
(Active and total bit rates for various formats).
The ATSC system employs multiple picture formats, digital audio and video compression. The compressed video and associated audio data streams are packetized into a packetized elementary stream (PES). One (i.e., one HDTV program) or several (i.e., multiple SDTV programs) PES together with auxiliary and control data and program and system information protocol (PSIP) are fed to a transport stream multiplexer, which combines them into a 19.38Mb/s data stream.
Translating into non-tech speak, the above paragraph means that you can only fit one HDTV program in the assigned channel (running at 19.38 Mb/s). That's good picture quality (HDTV). However, the broadcasters want to make some money off this, so they stuff more programs into the same space. How does that work? They drop the bit rate in each program, while the total still comes out to 19.38 Mb/s. Dropping the bit rate means less information is transmitted and the picture quality suffers.
Instead of one HDTV program you can fit several SDTV programs into the channel. Those who don't care about picture quality get more programs, and the broadcaster gets more revenue.
The idea is the same for cable or satellite TV. You still only have so many bits to stuff down the coax (or through the satellite). To fit more channels, the cable companies will drop the data rate on a few channels so they can add more channels and get more revenue. The data rate drop can be in the form of data compression (which is a different way to reduce PQ).
In any case, this is why ESPN may look sharp, while Discovery or some other channel may look soft. The cable operator is mucking around with the data rates to deliver high quality pictures on channels which generate the most revenue, at the expense of others.