It's quite simple why, the more republicans that vote against trump the easier it will be for Biden to win.

Politics has always been about choosing the possible over the perfect.

First off we need to remember this type of convention has not been done before so there will be problems.
2ndly there are republicans who are centrist minded and hate what tRump is doing to the country and the party. Look at the Lincoln Project. Many feel they cannot speak as the uber tRumbers will make life hard for them but now, at this critical time, they are speaking up. We cannot have a single party country! There needs to be opposing sides and a balance. With a monopoly we will get a one-sided approach and the democrats will become as power and money hungry as some of the republicans currently are.
This state had a very outspoken Republican named Slade Gordon. I hated him and wrote to him and the papers about his ramblings. But now he seem mild compared to what we are witnessing. He just died at 92 and there have been many stories on the local NPR stations about him and his rise and even some of the good things he did. One thing is that since tRump and his ilk took over Slade has been a very outspoken critic. Sometimes it seems, over time, politicians often lessen their rhetoric. [abcnews.go.com]

Corporate democrats are more like centrist republicans,bought by same lobby money,IMHO

Excellent political commentary about lighting and picture qualify from YouTube no Iess