Problem is not with ideology as it just a matter of priority for different parties.
Disparity in strength makes it hard to merge. Would WP wants to merge with with weaker parties like SPP, SDA or RP and let them ride on her coattail?
It just like if you are a A grade student, would you want to form project team with the C or E graders?
It would be easier for parties on same level to form alliance. Namely SPP, RDU, RV, SDA, PPP, RP.
Sdp and WP got their own branding and history. It would be silly to forgo all these attributes just to merge rename themselves.
Other than this, style of politics and character of individual also matters. WP requires potential candidates to do groundworks for a period before considering their candidacy. Things like distributing foods and organising durian tours does seems to be out of line with what many think opposition should do but it activities like this that enables WP to establish their roots in AJ GRC that pap finds it hard to pull out. Would people like KJ or even csj willing to engage in grassroots activities?