In Texas, 99.5% of all cases are resolved by plea bargain. Its all for the same reason, and innocence or guilt doesn't factor in: The prosecutor will offer a deal that you'd be a fool to reject unless you were certain you could win your trial.
If you happen to be poor, and in the right area, the court might be one that appoints public defenders without any fuss (that isn't a good assumption to make, though) or maybe you've got lucky and the area you are in has an actual public defenders office, in which each attorney is assigned a mere 200 cases at a time.
Its easy to say he was stupid for not taking the plea bargain, but innocent people take these plea bargains too, for the same reasons - felony record and probation or 5 years in prison? Sure you can win? Want to take the gamble?
Not my site, but one of a few that really covers this subject: http://gritsforbreakfast.blogspot.com/