It's been covered before, but it's still worth your time to read this op-ed by former Lt. Gov. Bill Hobby on how Texas came to adopt the law that restricted Republicans from replacing Tom DeLay on the ballot in CD22. Note the reason for the law:
In 1983 the Texas Legislature passed Senate Bill 122 to stop the parties from ignoring their own primary voters by removing nominees from the November ballot and replacing them with other nominees without the voters' approval.[...]
When SB122 was heard in the Senate State Affairs committee the sponsor (then-Sen. Kent Caperton, D-Bryan) summed up the need for the bill:
"[I]t clears a deficiency ... concerning the replacement of nominees in primaries. The current code allows the possibility that the general public has no say in who is the nominee of particular parties. In fact, we saw that occur last spring where not through ineligibility, not through unavailability to serve, or not through any other reason than political considerations."
The Senate's State Affairs chairman, Sen. Ray Farabee, D-Wichita Falls, was concerned that an unforeseen catastrophic illness would be a legitimate reason for a candidate's withdrawal but pointed out that a court should decide about that. Farabee wanted the illness to be documented by "providing for evidence to substantiate the illness so that it's there and determined, as compared to people who are getting together and deciding that, well, I don't feel too well, but it would be a lot better to run with this [other] candidate at this stage."
SB122 was reported out of committee and passed by the Senate with little opposition.