You're right; there is tremendous pressure from both district attorneys and judges to cop a plea. Trials piss everyone off because they are "such a waste of time." Criminal defense attorneys have to contend not just with the facts of the case and legal precedents, but also the extent to which demanding a jury trial turns the court against your client -- and your other clients, by association.

The other issue is police overcharging crimes. For example, suppose you, in a fit of pique, cut your roommate's arm with a knife. Not cool; that's aggravated assault. But the police then charges you with attempted second degree murder -- or even first degree (eg, premeditated). Now your defense attorney has to work hard just to get your charges down to a reasonable level; the agg assault you should have been charged with in the first place. This is what a huge majority of plea bargaining is about; not getting away with it, but getting the charge down to something that describes your actual offense.

(Source: longtime criminal defense nerd)