Why didn't the Umayyad caliphate which conquered vast tracts of land from Africa to Iran continue conquest towards East into the fertile lands of India and modern day Pakistan?

They tried. They lost miserably.

Contrary to what the intellectuals hold, the Gupta Emperors were around well into the 8th Century and likely held a substantial amount of Authority over the kings of Bharatavarsha. The end of the Gupta line was marked not by fragmentation of the Empire, but the constituent kings, guilds, and generals rallying primarily around Pratihara.

Thus until the 10th Century, Imperial Power in India was centred at Ujjaini under one Emperor, over a territory stretching from the Paripatra mountains in the North to Satpura in the South and from Amarkantak and Dwarbanga in the East to Makrana in the West.

This was the Power that faced off against the Peaceful hordes.

By the 8th Century, the Hunnic wars had been over 300 years in the past and it's likely that Arab style light cavalry warfare would've been a nightmare to deal with- not for the Heavily Armoured Bharata Armies, but for the common peasantry in the fields. Sindhu was hammered. Mulasthana- and the great Surya temple there- fell to the Mlecchas. There were raids at the very borders of Madhyadesha.

But ever have Bharata Arms, when led by one mind conversant with Dharma, been superior to the Mlecchas.

The 8th and 9th Centuries appear to have marked the final shift of Bharata military systems from the Bureaucractic norms fanoiref by the great Classical Empires to a more feudal-esque system. These forces were faster to raise, easier to handle, and more responsive to the Arab raiders. There were drawbacks to this- but that's a story for the 12th Century.

Arabs started losing armies, then territory. Their records are conspicuously silent on these years but it's a matter of record that visiting Peaceful scholars note how Peaceful rule in Bharatavarsha is limited to a single City- Mulasthana, surviving only because they'd threaten to destroy the Vigraha of Surya every time a Bharata army came over to give them their just desserts.

However it were Bharatas who had the last laugh. Sometime in the 9th Century, coins featuring the names of the Lords of Ujjaini begin turning up alongside local deals featuring Arab names. Some Malwa army finally had swept away the pathetic Peacefuls and restored Mulasthana to Aryarajaneya and Dharma.

Jai Shri Rama.

A century later, things took a turn for the worse- but that involved the Turashkas and not the Arabs, and thus is a story for another day.