The reason is simple.
The Islamic Trusts and the Waqf supervision will be Gone With the Wind and they will become paupers!
And all the pomp and show that the name Pataudi or Bhopal will go.
How many remember Duleep Singh and later in life nicknamed the Black Prince of Perthshire,who was the last Maharaja of the Sikh Empire?
He was the youngest son of the legendary "Lion of the Punjab" Maharaja Ranjit Singh and Maharani Jind Kaur
He is but a lost footnote in
Sikh history since he converted to Christianity.
Saif Ali is a mere actor and nothing more.
He survives celebrity status being the Nawab of Pataudi, which in any case, is a postage stamp size!
Will he give that postage stamp sized honour up to be a non entity?
If he should want to spend his time after his acting days are over and join politics as many has been actors have done, to be a Muslim has its advantages along with the name Nawab of Pataudi.
Therefore, it is wise decision that Kareena Kapoor changes to be a Muslim and anyway she is a Pathan (or whatever a woman Pathan is called) I believe.
She will blend like a duck to water.