Ghaznavi

This is a good question. Here are some reasons:

Decline of Rajput power in the 10th Century

Mahmud of Ghazni started his invasions at a time when Rajput power had declined for two reasons:

1. The empires of the Gurjar-Pratiharas and the Rashtrakutas had both declined into a bunch of smaller kingdoms. The Pratiharas are often credited with stopping the Caliphate so their fall was a blow.
2. The Rajputs had broken up into small feudal states called Samanthas. These states valued their independence more than the solidarity they shared with each other. So for one it was harder to get these armies together than it was for the Turkic Sultans with "slave" armies like Mahmud's (note the word slave is in quotes). Then when they did get together they fought as separate armies. This problem would plague the Rajputs well into the Mughal era, until their eventual d
   Military Tactics

 Now Mahmud did not just romp through "India". He failed in his attempts to take Kalinjar, Gwalior and Lokote. Also after his well-known sack of Somnath he returned through the Thar desert to avoid the armies of Ajmer and their allies. At times he was just too fast for the Rajputs to get together and take him on.

17 is 17 because many of his "invasions" were hit-and-run attacks, albeit at a grand scale. Timur invaded India only once - but no one thinks he was inferior to Mahmud.

Now if the Rajput empires had remained, they would have had the resources to adjust to the new paradigm of war which the Turkic invaders were bringing in. These invaders depended on fast moving cavalry as against the Indian armies that were based around the elephants. To their credit the Rajputs did evolve, so that in Mughal times the Rajput cavalry was much prized by the Mughals. Still in the 11th Century, this evolution was not fast enough to keep pace with this new invasion which brought superior military tactics and also the fundamentalist fervor of the newly converted (during Mahmud's time even Afghanistan still had a lot of Hindus and Buddhists).

Now Mahmud's biggest achievement was establishing a base for the Turkic Sultans East of the Hindukush. For 150 years after his death, his successors were much weaker than him and often fought among themselves. Yet the Rajputs of North India kept suffering their raids, not really focussed on kicking them out. Eventually Muizzuddin Mohammed of Ghori would unite the Turkic tribes and come and settle them down in Delhi for goodecline into irrelevance during the Maratha period