I suspect any answers would be fairly complex and would be shaped by history and a national character which views external rule - secular or spiritual - with some degree of suspicion.
Historically, the faith was outlawed until Catholic emancipation in 1829. Even then, the burdens placed on Catholics and the rules preventing them from participating fully in British society were not completely lifted and continue to this day.
To be Anglican was to be part of the very fabric of English society - it is the established Church and wields a great deal of power. To be Catholic was to be part of a marginalised and hated minority.
Growing up in a society where Catholics were generally considered to be duplicitous, work-shy troublemakers and where the only discussions of Christian doctrine are framed in an Anglican context, most candidates for holy orders wouldn't likely consider Catholicism. Initially, at least.
The Catholic Church in England hasn't exactly covered itself with glory either since the 60s and is widely perceived as being further advanced towards apostasy than the Anglican Church. gαy masses, sodomite archbishops, scandalous behaviour among the priesthood may have persuaded some that it was better to stick with the devil they know.
Archbishop Nichols has been less than welcoming to the Ordinariate - offering them little in the way of support and mouthing off in public about his hopes for continued ecuмenical fraternisation and deploring privately Benedict XVI's olive branch to traditional Anglicans. He's a well-known wolf within the sheep-fold and a scarier prospect than the last few Archbishops of Canterbury - weak and gutless characters who prefer to do nothing rather than rock any boats.
Many Catholics have also observed that high Anglicanism preserved reverence and purity of ritual rather better than the NO Church cared to. The Anglican Church always had its Kumbaya wing, but it has always been easy to avoid them.
Even with the introduction of women priests, traditionalist Anglicans could avoid the political agenda as they were not permitted to minister to traditional parishes. Everything has now changed with the new Archbishop of Canterbury and the likely introduction of women bishops and acceptance of gαy 'marriages'.
I'd guess that many of those in the Ordinariate have agonised long and hard about separating themselves from a church which has always celebrated the art of compromise. They've probably reached a point where they've realised that it's just not possible to compromise with a runaway train headed for the buffers at 130mph.
Converts like Robert Hugh Benson, the vicar son of an Archbishop of Canterbury, have historically been few. His autobiography is a fascinating study of the journey from Canterbury to Rome and well worth reading to understand what an Anglican of conscience goes through in choosing to make that journey.