Consider the kind of person who’d want to join the Opposition. First of all, it means you’ll be picking a fight with a ruling party that is beloved by the majority of the population. It’s a tough fight that you’re almost surely going to lose. Why would you do that?

Here are some of the reasons I can think of:

1. You have deep principles. You’re convinced that there’s something wrong that most people are missing, and you find it necessary to fight to support for your POV. You might hate conflict but feel that you have no choice.
2. You’re fundamentally disagreeable. (Remember, this isn’t necessarily a bad thing. People like this have achieved tremendous things in human history.) Maybe you thrive off of conflict itself. If somebody puts forth a proposal, you find yourself compelled to point out flaws and argue against it.
3. You want attention and authority. You feel validated by wearing your party colors, you like walking around and shaking hands, and you like the idea of maybe having a shot at being an MP or a Minister or someone with status.

Notice that all 3 of these attributes create contexts for conflicts within any Opposition party. If we disagree with something on principle, I’m going to leave and start a party that is aligned with my principles. If we just disagree for the heck of it, screw you, I’m starting my own party. And why should I have to be your #2, when I can be my #1?

TLDR: Opposition parties are populated by people who, for better or worse, like to oppose. This makes it hard for them to come to internal agreements. If they were so agreeable, they’d never have joined an Opposition party in the first place – they’d have joined the PAP instead.