Q:

Why was kidnapping such a prevalent custom of marrying women in ancient aryavarta?

Bhishma abducted women:

Meanwhile the son of the ocean-going Ganga heard that king Devaka had a daughter endued with youth and beauty and begotten upon a Sudra wife. Bringing her from her father's abode, Bhishma married her to Vidura of great wisdom. And Vidura begot upon her many children like unto himself in accomplishments.' 

-- An excerpt of Adi Parva Section CXIV
Krishna abducted women: 

Of eyes like the lotus petals, and endued with great bravery, Krishna, vanquishing all the kings at a self-choice, bore away the daughter of the king of the Gandharas. Those angry kings, as if they were horses by birth, were yoked unto his nuptial car and were lacerated with the whip.

-- Excerpt of Drona Parva Section XI
Duryodhana abducted women: 

Disregarding all the kings, he commanded the maiden to stop. Intoxicated with the pride of energy, and relying upon Bhishma and Drona, king Duryodhana, taking up that maiden on his car, abducted her with force. 

Excerpt of Shanti Parva Section IV
Why was kidnapping such a prevalent custom of marrying women in ancient aryavarta?

A:

In scriptures 8 types of marriages are described.

Manu Smriti 3.21. (They are) the rite of Brahman (Brahma), that of the
  gods (Daiva), that of the Rishis (Arsha), that of Pragapati
  (Pragapatya), that of the Asuras (Asura), that of the Gandharvas
  (Gandharva), that of the Rhashasas (Rakshasa), and that of the Pisakas
  (Paisaka)

Also, the following verses describe which marriages are suitable for which Varnas.

3.23. One may know that the first six according to the order (followed above) are lawful for a Brahmana, the four last for a Kshatriya, and
  the same four, excepting the Rakshasa rite, for a Vaisya and a Sudra.
3.24. The sages state that the first four are approved (in the case) of a Brahmana, one, the Rakshasa (rite in the case) of a Kshatriya,
  and the Asura (marriage in that) of a Vaisya and of a Sudra.

And, in general,

3.25. But in these (Institutes of the sacred law) three of the five (last) are declared to be lawful and two unlawful; the Paisaka and the
  Asura (rites) must never be used

Now,
Gandharva marriage:

3.32. The voluntary union of a maiden and her lover one must know (to be) the Gandharva rite, which springs from desire and has sexual
  intercourse for its purpose.

Rakshasha marriage:

3.33. The forcible abduction of a maiden from her home, while she cries out and weeps, after (her kinsmen) have been slain or wounded
  and (their houses) broken open, is called the Rakshasa rite.

Pishacha marriage:

3.34. When (a man) by stealth seduces a girl who is sleeping, intoxicated, or disordered in intellect, that is the eighth, the most
  base and sinful rite of the Pisakas.

So, when both the bride and bridegroom have no problems in marrying, that marriage comes under the Gandharva Rite. This one is acceptable even if kidnapping is involved.
However, the Rakshasa and Pisacha rites, where marriage takes place without the approval of the maiden and with force or stealth being involved, are generally considered as blameable forms of marriages.
For Kshatriyas, however, the Rakshasa marriage is approved as you can see from one of the verses given above.

